Step-sections of 429 whole prostate glands were studied. Large acinar atypical hyperplasia was graded as mild, moderate, and severe based on the degree of cellular anaplasia. The relationship between atypical hyperplasia and prostatic carcinoma was investigated. There was a strong association between prevalence and grade of atypical hyperplasia and prostatic carcinoma in patients up to 60 years old. Beyond the age of 60 no association was detected. In the younger age groups (36 to 60 years) atypical hyperplasia was found in 86.8% of prostates with carcinoma, but only in 37.9% of benign glands. Corresponding figures for the over-60 age group were 68.8% and 65.1%, respectively. A biologic explanation of the association between atypical hyperplasia and carcinoma in the younger age groups has been proposed. It was suggested that these men with atypical hyperplasia, particularly with severe atypical hyperplasia, have a greater risk for developing prostatic carcinoma.