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Background. Echinacea is a herbal preparation that is frequently used to treat the common cold. Spending on echinacea in the United States has risen to >$300 million annually. Methods. A total of 322 articles related to echinacea and colds, including 9 placebo-controlled clinical trials, were identified using the Medline and PubMed databases. Eleven features of experimental design that affect the accuracy of the measurement of features of interest, the probability of a chance relationship, bias, and blinding were used to evaluate the 9 placebo-controlled studies. The criteria were validated case definition, quantifiable hypothesis, sample-size calculation, randomized assignment, double blinding, proof of blinding, measurement of compliance, measurement of drop-out rate, analysis by intention to treat, description of the methods of analysis, and measurement of probability. Equal weight was given to each criterion, since failure to meet any one of them could potentially invalidate the findings of a clinical trial. Results. Of the 9 studies, 2 met all 11 criteria. The results of both studies were judged to be negative by the people who performed the studies. Of the remaining 7 studies, 6 were judged to have positive results, and 1 was judged to have negative results. The criterion most commonly not met was proof of blinding. Conclusions. This structured review suggests that the possible therapeutic effectiveness of echinacea in the treatment of colds has not been established. [ABSTRACT FROM AUTHOR]