In the Netherlands Cohort Study, the authors investigated whether anthropometry is associated with prostate cancer risk. At baseline in 1986, 58, 279 men aged 55–69 years completed a self-administered questionnaire on diet, anthropometry, and other risk factors for cancer. After 6.3 years of follow-up, 681 cases were available with complete data on height and weight at baseline, and for 523 cases, there were data for weight at age 20 years. In both age-adjusted and multivariate case-cohort analyses (adjusted for age, family history of prostate cancer, and socioeconomic status), height, body mass index (BMI; kg/m2), and lean body mass (kg) at baseline were not associated with prostate cancer risk. The rate ratios of prostate cancer for men with a BMI at age 20 of less than 19, 19–20.9, 21–22.9, 23–24.9, and 25 or greater were 1.00 (reference), 1.06, 1.09, 1.39, and 1.33, respectively ( p for trend = 0.02). For gain in BMI from age 20 years to age of the cohort at baseline, an inverse trend in risk was found ( p for trend = 0.01), which did not persist after additional adjustment for BMI at age 20 ( p for trend = 0.07). In subgroup analyses, no clear associations between anthropometry and advanced prostate cancer were found. Our findings suggest that body composition in young adulthood may already exert an effect on later risk of prostate cancer. Am J Epidemiol 2000;151:541–9.