In an effort to better distinguish the morphologic relationship of atypical hyperplasia of the prostate to benign prostatic hypertrophy and prostatic cancer, 43 prostate specimens were analyzed with ten immunohistologic markers. Two cytokeratin antibodies appeared useful (Cyto M and Cyto P, with the latter slightly more discriminatory). In summary, it appears that atypical hyperplasia is immunohistopathologically related to both benign prostatic hypertrophy and prostatic cancer, having characteristics of both.