The identification of basal cells is often helpful in excluding a diagnosis of prostate carcinoma. However, it can be difficult to distinguish basal cells from underlying fibroblasts or an artifactual two-cell layer in neoplastic glands. To determine the usefulness of anti-keratin antibody 903 for identifying basal cells in glandular patterns sometimes confused with carcinoma, we examined frozen sections from radical prostatectomy specimens and formalin-fixed needle biopsy, radical prostatectomy, and transurethral resection specimens. Atrophic glands, basal cell hyperplasia, intraductal severe dysplasia and various grades of carcinoma were examined. Also evaluated were cases of atypical adenosis, defined as clusters of small glands that mimic low-grade carcinoma yet focally appear to have a basal cell layer and merge with more recognizable benign glands. Almost all normal glands showed some staining, although it was often discontinuous with formalin fixation. Intraductal dysplasia stained in a manner similar to normal glands. Ninety-two percent of atrophic glands and 88% of glands in basal cell hyperplasia stained. Sixty-one percent of the glands in atypical adenosis stained intensely but discontinuously. All grades of adenocarcinoma lacked any immunoreactivity. These results indicate that keratin 903 is useful in the diagnosis of prostatic carcinoma because positive staining identifies a questionable focus as benign whereas negative staining helps to substantiate the diagnosis of carcinoma.