Background: Carcinoma of the prostate is the second most common cause of cancer deaths in men. Little is known about the pathogenesis of this disease and the molecular genetic events that contribute to its development. Molecular studies have begun to reveal biologic characteristics of this disease, notably, the loss of genetic material as determined by studies of restriction fragment length polymorphism, oncogene activation, and production and response to growth factors. Purpose: Our goal was to characterize p53 gene mutations in human carcinoma of the prostate and to analyze base-pair changes within the coding regions of p53 mRNA (exons 4 through 11). Methods: Forty-four prostate tissue specimens and four metastatic lesions were obtained from 48 prostate carcinoma patients who had surgical resection. RNA was either immediately extracted or the specimens were snap-frozen in liquid N2 and stored at −70 °C until used. Total RNA was extracted from tumor specimens. Expression of p53 was analyzed by polymerase chain reaction (PCR) analysis of mRNA (RNA/PCR). Following confirmation of the RNA/PCR products by Southern blotting, quantitation of message levels was performed by laser densitometry. Absolute area integrations of the curves representing each tissue were then compared after adjustment for the housekeeping gene c-N-ras. Two overlapping regions (exons 4–6 and 6–11) were examined by a nonisotopic PCR single-strand conformation polymorphism (SSCP) analysis system. All specimens displaying SSCP abnormalities were sequenced in both directions to confirm the findings. Results: Of the 48 prostate specimens, three (6%) (two primary and one metastatic) displayed nearly undetectable expression of p53 mRNA (samples PS-70, L113, and PS-95) and 17 (35%) of 48 expressed mutant p53 mRNA encoding amino acid substitutions within exons 4–11 (14 of 17) and/or deletions within the p53 transcripts (three of 17). Overall, the frequency of p53 gene abnormalities that would result in altered protein expression was 20 (42%) of 48 in the tissue samples from prostate carcinoma patients. Nucleotide base-pair transitions of A->G or T—>C were the most frequent Conclusions: p53 mutations are common in prostate cancer. The patterns of p53 gene mutations are dramatically different from data obtained on other cancers and indicate the possible involvement of a carcinogenic agent(s). Implications: Further studies are required to determine the biologic role of p53 gene alterations in the development and progression of this disease and to determine whether p53 mutations can be useful as prognostic markers or for the selection of better treatments for prostate cancer patients. [J Natl Cancer Inst 86:926–933, 1994]