We conducted a meta-analysis of the association between prostate cancer and aspects of sexual activity. The data suggest an elevated relative risk (RR) of prostate cancer among men with a history of sexually transmitted infections. This was observed with both random- and fixed-effects models (RR = 1.4; 95% CI = 1.2-1.7; N = 17 studies; heterogeneity P = 0.14), especially for syphilis (RR = 2.3; 95% CI = 1.3-3.9; N = 6; heterogeneity P = 0.47). Risk of prostate cancer is also associated with increasing frequency of sexual activity (RR = 1.2 for an increase of three times per week; 95% CI = 1.1-1.3; N = 12). However, these studies are heterogeneous (P < 0.001). Increasing number of sexual partners is also associated with prostate cancer (RR = 1.2 for an increase of 20 partners; 95% CI = 1.1-1.3; N = 16; heterogeneity P = 0.11). The data do not support associations with multiple marriages, age at first intercourse, or age at first marriage. These results indicate an association between prostate cancer and sexually transmitted infections, suggesting that infections may represent one mechanism through which prostate cancer develops. The mechanism through which frequency of sexual activity may be related to prostate cancer is unclear.