Hyperplastic lesions of the prostatic stroma with atypical changes are presented and discussed. One lesion was a leiomyoma with atypical organization and a limited area of nuclear pleomorphism without mitotic figures. One was a lesion which in many portions was histologically indistinguishable from leiomyosarcoma. It, however, had certain features probably connoting less sinister behavior, and at autopsy 16 months after resection there was no evidence of recurrence or metastasis. One lesion histologically resembled fibroadenoma of the female breast. These three lesions occurred in patients more than 50 years who had symptoms and signs of prostatism. Reference is made to two previously reported lesions reminiscent of cystosarcoma phyllodes of the female breast. All the lesions were located in the craniad prostate, which is estrogen-influenced. A parallel is drawn between these atypical changes in prostatic fibromuscular stroma and atypical changes in the connective tissue stroma of the female breast.