Prostatic basal cell proliferations range from focal basal cell hyperplasia (BCH) to florid adenoid basal cell tumor (ABCT). We reviewed 36 cases of basal cell proliferation to evaluate the architectural and cytologic spectrum of these lesions and identified four distinct patterns by light microscopy: BCH, 12 cases, including two cases with stromal sclerosis; atypical BCH with nucleolomegaly, eight cases; basal cell adenoma (BCA), nine cases; and ABCT, seven cases. Twenty-three cases were evaluated immunohistochemically, and all displayed cytoplasmic immunoreactivity in basal cells with basal-cell-specific high-molecular-weight keratin 34 beta E12 but with increased staining in BCH compared with that in BCA and ABCT. Prostate-specific antigen and prostatic acid phosphatase reactivity were seen at least focally in all cases except two cases each of BCA and ABCT. Chromogranin, S-100 protein, and neuron-specific enolase reactivity were rare or negative in all cases. Nucleolar diameter, measured in 18 of the 36 cases, was significantly greater in atypical BCH (mean, 1.96 microns) than in other forms of basal cell proliferation (mean, < 1.0 microns) (P < 0.05). These results indicate that on the basis of light microscopic, immunohistochemical, and morphometric findings, the spectrum of basal cell proliferations in the prostate can be separated into four distinct groups: BCH, atypical BCH, BCA, and ABCT.