The posterior lobe of the prostate of 180 consecutive autopsies was examined by the step section method. Latent carcinoma was found in 25 per cent of the prostates examined, with an increasing frequency according to age. Atypical lesions were seen in 153 of 180 prostates (85%) and were significantly more frequent and extensive in patients with rather than without latent carcinoma. Latent carcinoma was more extensive either when grade 2 histologically or when showing cribriform arrangement. There was no relationship of latent carcinoma to nodular hyperplasia or extraprostatic cancer. Atypical lesions were not significantly more extensive in patients with large latent carcinomas than in those with small ones. Atypical lesions als showed no significant increase in frequency according to age. The findings support the concept that atypical lesions may represent a potentially precancerous lesion.