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Recent epidemiologic studies of chronic diseases have necessitated the development of dietary methods that can be used with large population samples, at relatively low cost, while still yielding valid results. One of the simplest approaches for measuring dietary intakes is to ask only about the frequency with which foods are eaten over a specified time period. The advantages of the frequency dietary methods are obvious. They can be administered cheaply and quickly to large numbers of people, and the numerous problems associated with the collection of quantitative dietary data are avoided. A number of studies reported in the literature have relied on frequency dietary data, occasionally translated into quantitative values by statistical conversions, to investigate possible associations between diet and disease. However, it is not known whether frequency dietary data can be used in place of the more laboriously collected quantitative dietary data without affecting the validity of the results. The few attempts to validate frequency dietary methods against traditional quantitative dietary methods (e.g., Burke dietary histories, food records) have been limited by the size of the sample and cursory data analyses. The main objective of the present study was to determine whether frequency dietary intake data can substitute for quantitative intake data in epidemiologic studies without affecting the diet-disease associations measured. This was done by determining the extent of agreement between individual frequency and quantitative dietary intake assessments collected from 342 male subjects participating in a case-control study concerning diet and prostate cancer risk. The dietary information was obtained using a recall interview method designed to assess the subject's past, usual dietary habits. The questionnaire consisted of a list of 113 foods, selected to cover the major sources of dietary fat, vitamin A, zinc, and cadmium. The, frequency and quantitative amount of intake were recorded separately for each food item eaten. Quantitative amounts were estimated using photographs of small, medium, and large portions of each item. Since the precision of data required varies according to the objectives of a particular investigation, the extent of agreement between frequency and quantitative measurements was examined for each dietary factor (44 food items, 20 food groups, and 8 nutrients were evaluated), at both the group and individual level, and with regard to the specific study objective. If the study objective involves the comparison of the intakes of individuals or groups, e.g., case-control, cohort, or aggregate correlational studies, then relative agreement is sufficient to determine the interchangeability of frequency and quantitative dietary intake measurements. In other words, the correspondence between the two measurements does not have to be exact, but it must be consistent among different subjects (or groups) to allow the substitution of frequency for quantitative dietary data without changing the measured diet-disease association. If, however, information on the actual amount consumed is required, e.g., in metabolic balance studies, then absolute agreement (intakes based on the two measurements correspond exactly in magnitude) is necessary to demonstrate interchangeability between frequency and quantitative intake data. At the aggregate level, frequency and quantitative dietary measurements were found not comparable for estimating the exact amount (absolute agreement) of mean dietary intakes of food items, food groups, or nutrients. There was a moderate degree of relative agreement between the mean intakes of food items (the two approaches ranked five ethnic groups on mean intakes equally (τ= 1.0) for 43% of the food items tested), but not for food groups (τ = 1.0 for 15% of the food groups tested). For the nutrients, relative agreement between mean intakes based on the two measurements was even lower. This suggests that frequency and quantitative dietary measurements can be used interchangeably to rank groups by mean intakes of certain food items (e.g., in ecological analyses), but not of food groups or nutrients. At the individual level, there was essentially no absolute agreement between the two types of dietary measurements for any dietary ,factor. There was a moderate degree of relative agreement between individual frequency and quantitative intake measurements, but because of the somewhat limited nature of the quantitative measurements collected, the findings were interpreted conservatively. In general, the extent of agreement was insufficient for one to be confident that frequency intake data can replace quantitative data without affecting the statistical findings. This suggests caution in interpreting the findings of epidemiologic studies that have used frequency of intake data to estimate quantitative dietary intakes, especially when individual intakes are compared. Frequency dietary data, however, may be useful in exploratory stages of investigating diet-disease associations, especially when the dietary hypotheses concern specific food items.