Fas (Apo-1/CD95) is a cell-surface receptor involved in cell death signaling through binding of Fas ligand. Mutations of the Fas gene might be involved in proliferative diseases of the prostate by prolongation of programmed cell death of prostatic epithelial cells. Using the laser capture microdissection method, Fas gene mutations were examined on genomic DNA extracted from lesions with high-grade prostatic intraepithelial neoplasia (HGPIN), a possible precursor of prostatic cancer (PCA), and from PCA. A total of 193 lesions, 111 with HGPIN, 55 with PCA, and 27 benign glands, were microdissected from 27 patients with PCA. Polymerase chain reaction-amplified products were directly sequenced. Loss of heterozygosity (LOH) was examined at four sites of known polymorphisms. Fas gene mutations were detected in HGPIN: 4 of 27 (14.8%) cases or 4 of 111 (3.6%) lesions. All were point mutations: three missense and one nonsense in the death domain. Benign proliferative glands adjoining HGPIN and/or PCA, and PCA never showed mutations. LOH was found in 31.3% of PCA and 25% of HGPIN lesions, but was never found in benign glands. Exclusive occurrence of Fas mutations in HGPIN might underlie the development of these lesions. Occasional findings of LOH in HGPIN and PCA suggested that genetic instability might occur during the early phase of prostatic carcinogenesis.