High intake of cruciferous vegetables may offer some protection against prostate cancer, but overall data are inconclusive. Thus, we examined the association between cruciferous vegetable intake and risk of prostate cancer in the Health Professionals Follow-Up Study. Between 1986 and 2000, 2,969 cases of nonstage T1a prostate cancer were diagnosed in 47,365 men who completed dietary assessments in 1986, 1990, and 1994. We calculated the multivariate relative risk (RR) and 95% confidence intervals (CIs) using Cox regression. Overall, we found no appreciable association between baseline intake of cruciferous vegetables and risk of prostate cancer (RR, 0.93; 95% CI, 0.82-1.05, for > or = 5 versus < or = 1 serving/week; P for trend = 0.30), and only a slight suggestive association for organ-confined prostate cancer (RR, 0.88; 95% CI, 0.74-1.05; P for trend = 0.06). The inverse association was stronger for men under the age of 65 years (RR, 0.81; 95% CI, 0.64-1.02; P for trend = 0.02), especially for organ-confined cancers (RR, 0.72; 95% CI, 0.54-0.97; P for trend = 0.007). In addition, this inverse association was stronger when we restricted the analysis to men with more consistent intake of vegetables over the 10 years before 1986, when we limited the analysis to men who had had a prostate-specific antigen test, and when we considered an 8-year time lag. This study does not provide compelling evidence of a protective influence of cruciferous vegetables on prostate cancer risk. However, if cruciferous vegetables are protective early in prostate carcinogenesis, as suggested by proposed mechanisms, we may expect stronger associations, as observed, for more remote diet for prostate-specific antigen-detected early stage (organ-confined) cancers in younger men. In contrast, for advanced cancers in older men, which were probably initiated decades in the past, recent dietary intakes of cruciferous vegetables may be irrelevant. These findings suggest that future studies of cruciferous vegetables should focus on early stages of prostate cancer.