Recent literature reports inverse associations with ejaculator frequency and prostate cancer (PC). We sought to explore the relationship between ejaculatory frequency from ages 20 to 50 and subsequent development of aggressive PC.We conducted a case-control study sampling 2,141 men from private urology practices in Victoria, Australia. Cases were defined as men with high grade or high stage PC and controls being biopsy negative men. Ejaculation frequency recalled at age decades 20, 30, and 40 second was assessed by questionnaire. Unconditional multivariable logistic regression models were used to generate odds ratios (ORs).An inverse association with ejaculatory frequency at age 30 to 39 was observed (OR per 5-unit increase per week = 0.83, 95% CI: 0.72-0.96) but not at ages 20 to 29 (OR = 1.01, 95% CI: 0.89-1.14) or ages 40 to 49 (OR = 0.95, 95% CI: 0.81-1.12). This result differed between men with new sexual partners after age 30 (OR = 0.77, P = 0.009) and those with no new partners (OR = 0.97, P = 0.8) though the test for a difference between these estimates was not significant (P = 0.11).We found only weak evidence of an inverse association between ejaculatory frequency in the fourth decade of life and advanced PC, which was not significantly modified by number of new sexual partners. No relationship was found for ejaculatory frequency in the third and fifth decades of life.