Basaloid proliferations of the prostate with morphologic patterns other than usual basal cell hyperplasia are rare, and the distinction between benign and malignant lesions has been difficult. We describe 23 such lesions and classify them into two groups: adenoid cystic-like hyperplasia and adenoid cystic or basaloid carcinoma. Adenoid cystic-like hyperplasia (n = 19) was characterized by an older age at presentation (mean, 71.8 years), transition zone location with background of nodular hyperplasia, multifocality, lobulation, circumscription, and small acini with occasional hyalinization. A cribriform pattern limited to small- and medium-sized glands, squamous metaplasia, and hypercellular myxoid stroma were occasionally seen. Adenoid cystic carcinoma (n = 3) was characterized by a younger age at presentation (mean, 46.0 years), peripheral zone involvement, and large acini that were often dilated and exhibited extensive interanastomoses, prominent intraglandular hyalinization, perineural invasion, and extraprostatic extension. Basaloid carcinoma (n = 1) showed infiltration between normal glands, perineural invasion, and extraprostatic extension but lacked a cribriform architecture. The degree of cytologic atypia and mitotic rate overlapped between the hyperplasia and carcinoma cases. Both hyperplastic lesions and adenoid cystic carcinomas showed a basal cell phenotype with strong immunoreactivity to cytokeratins 14 and 34betaE12, but the basaloid carcinoma was negative for these markers. In all cases, the proliferating basal cells were nonreactive for myoepithelial and prostatic secretory cell markers. The 8 patients with adenoid cystic-like hyperplasia with available follow-up information had no progression of disease (mean follow-up period, 8.6 years). One patient with adenoid cystic carcinoma died with widespread metastases, but the 3 other patients with carcinomas had no disease progression (mean follow-up period, 7.0 years). In conclusion, most florid basaloid proliferations of the prostate fall into one of two categories. In the first, there is a clear association with nodular hyperplasia (adenoid cystic-like hyperplasia) and, although cytologic atypia and mitoses may be seen, they are present within a lesion that retains an orderly, vaguely nodular (noninfiltrative) pattern. The second group of cases (adenoid cystic and basaloid carcinoma) shows a widespread, haphazard infiltrative growth pattern. This study suggests that adenoid cystic carcinomas are biologically indolent following prostatectomy but have a low risk of distant metastasis.