In a follow-up study of a cohort of 50-year-old men who were born in 1913 and were living in Gothenburg, Sweden, we found an association between birthweight and prostate cancer. Of 366 men with known birthweight, there were 21 patients with prostate cancer. The incidence is about five times higher in the highest quartile of birthweight than in other birthweight groups. The findings indicate that aspects of the pre- and perinatal period may affect the risk of subsequent prostate cancer.