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Because an inverse relationship between serum 25-hydroxyvitamin D (25[OH]D) levels and the risk of prostate cancer has been suggested, it was hypothesized that vitamin D insufficiency would be common in patients with prostate cancer. To test the hypothesis, an exploratory study was conducted to examine serum 25(OH)D levels in a cohort of patients with nonmetastatic prostate cancer. The study aim was to assess the prevalence of vitamin D insufficiency in these patients. Vitamin D insufficiency was defined as serum 25(OH)D less than 75 nmol/L. Serum 25(OH)D levels measured prospectively at baseline and, then, yearly during a 5-year follow-up were analyzed. Various parameters were examined to assess their possible association with vitamin D insufficiency at baseline, using both a univariate analysis and a logistic regression model. Analyses including descriptive statistics for all variables were carried out with SAS version 9.1 (SAS Institute, Cary, NC). A total of 106 patients were available for analysis. The median age was 66.3 years. At baseline, mean and median vitamin D level was 72.4 and 70.0 nmol/L, respectively. Sixty-four patients (60.4%) met the definition of vitamin D insufficiency with serum 25(OH)D less than 75 nmol/L. Forty (37.7%), 20 (18.9%), and 2 patients (1.9%) had serum 25(OH)D less than 62.5, less than 50, and less than 25 nmol/L, respectively. On a logistic regression model, season was the only significant variable associated with vitamin D insufficiency. Of a total 477 serum 25(OH)D measurements from the baseline and yearly follow-ups, 187 (39.2%) met the definition of vitamin D insufficiency. In conclusion, vitamin D insufficiency was prevalent among patients with nonmetastatic prostate cancer., (Copyright © 2011 Elsevier Inc. All rights reserved.)