Prevalence of carcinoma in situ and other histopathological abnormalities in testes from 399 men who died suddenly and unexpectedly. To determine the prevalence of carcinoma in situ of the testis and other testicular histopathological abnormalities in the general male population, we examined gonads from 399 men 18 to 50 years old who died suddenly and unexpectedly. No sign of malignancy was found in any of these gonads. However, 3 of the 399 men had been previously treated for testicular tumor or carcinoma in situ. Thus, the over-all prevalence of testicular neoplasia in the population studied was 0.8% (95% confidence limits 0.2 to 2.2%). This frequency is of the same magnitude as the lifetime risk of testicular cancer in the Danish male population. The median weights of the left and right testes were 19.3 and 19.7 gm., respectively. This difference was statistically significant (p = 0.00003). Thus, our study confirmed that on average the left testis is smaller than the right testis. The median weight of the gonads collected in our study was 0.9 gm. lower than the weight of testes examined 40 years earlier at the same department of forensic medicine. However, this difference was not statistically significant (p = 0.17). Microscopic examination of the gonadal specimens revealed that 83% of the men exhibited complete spermatogenesis, including late spermatids in all tubules. In the age group studied we found no age-related changes in testicular weight or in the proportion of tubules with degenerative changes, such as spermatogenic arrest, the Sertoli-cell-only syndrome or hyalinization. 