Basal cell hyperplasia classically has been described as having bland cytologic features. During the past 2 years, we have seen 12 cases (11 in consultation) with atypical features that were confused with adenocarcinoma of the prostate. Eleven of these 12 cases contained prominent nucleoli mimicking carcinoma; in the 12th case, nuclei were enlarged, hyperchromatic, and moderately pleomorphic. Immunohistochemistry with antibodies against high-molecular-weight cytokeratin (34 beta E12) was performed in nine of the cases, verifying their basal cell nature. Additional findings in these cases were necrotic intraluminal secretions (two cases), immature squamous metaplasia (two cases), peculiar hyaline cytoplasmic globules (two cases), adenosis (one case), markedly atypical nuclei of uncertain nature occurring elsewhere in the specimen (one case), and intraluminal blue mucin (two cases). We analyzed nine cases of typical basal cell hyperplasia, all of which showed classic features of basal cell hyperplasia with benign cytology. Both atypical and classical basal cell hyperplasia were frequently infiltrated by lymphocytes such that the cytologic changes could not be attributable to inflammation. Atypical basal cell hyperplasia must be differentiated from ordinary adenocarcinoma of the prostate, prostatic intraepithelial neoplasia, and basaloid carcinoma (adenoid cystic carcinoma) of the prostate.