Sarcomas and related proliferative lesions of the specialized prostatic stroma have been the subject of case reports and, thus, have not been well characterized. We reviewed the clinicopathologic features of 22 cases and studied the immunohistochemical profile of 9. Patient age ranged from 25 to 86 years; mean age was 54 years, and peak incidence was in the 6th and 7th decades. The most common clinical presentation was urinary retention, then abnormal results of digital rectal examination, hematuria or hematospermia, and a palpable rectal mass. The cases were grouped into two categories: prostatic stromal proliferation of uncertain malignant potential (PSPUMP, 18 cases) and prostatic stromal sarcoma (PSS, 4 cases) based on the degree of stromal cellularity and the presence of mitotic figures, necrosis, and stromal overgrowth. Four histologic patterns of PSPUMP were identified: (1) hypercellular stroma with scattered cytologically atypical cells associated with benign glands, (2) hypercellular stroma with minimal cytological atypia associated with benign glands, (3) hypercellular stroma with or without cytologically atypical cells, associated with benign glands in a "leaflike" growth pattern that resembled phyllodes tumors of the mammary gland, and (4) hypercellular stroma without cytologically atypical stromal cells and without glands. Prostatic stromal sarcoma showed greater cellularity, mitoses, necrosis, and stromal overgrowth than PSPUMP and consisted either of stromal elements with benign glands in a pattern that resembled malignant phyllodes tumors of the mammary gland (3 cases) or of purely stromal elements (1 case). Positive immunohistochemical reactions were noted using vimentin in 9 of 9 cases, CD34 in 8 of 8, HHF-35 in 2 of 8, smooth muscle actin in 3 of 9, desmin in 4 of 8, S-100 protein in 0 of 9, estrogen receptor in 1 of 7, and progesterone receptor in 6 of 7. None of the cases classified as PSS were positive for HHF-35, smooth muscle actin, or desmin. Of the 13 patients classified as having PSPUMP who did not undergo definitive local therapy at the time of diagnosis, recurrent signs or symptoms were seen in six (46%), necessitating additional therapy. Distant metastases to lung and bone developed in one patient classified as having PSS. Clinical and pathologic findings in this patient suggested a progression from PSPUMP to PSS. We conclude that sarcomas and related proliferative lesions of the specialized prostatic stroma encompass a spectrum of histologic features and may be grouped into two clinicopathologic categories: PSPUMP and PSS. Based on their distinctive histologic appearance and immunohistochemical profile, PSPUMP and PSS can be differentiated from other mesenchymal lesions of the prostate.