Immunohistochemical staining for the p53 protein was performed in microwave-fixed, paraffin-embedded sections of normal, premalignant and malignant tissues of the female genital tract using a monoclonal antibody, PAb 1801. No staining was detected in normal and premalignant tissues, whereas nuclear staining of cancer cells was observed in 12 (22%) of 55 cervical squamous cell carcinomas, 4 (25%) of 16 cervical adenocarcinomas, 37 (42%) of 88 endometrial carcinomas, 23 (38%) of 60 ovarian adenocarcinomas, and 6 (100%) of 6 squamous cell carcinomas arising in dermoid cysts. Of interest, 1 of 7 endometrial cancers with concomitant atypical hyperplasia showed weak nuclear staining in a few atypical hyperplastic glands in addition to the cancerous lesions. Although staining was associated with cancers having a high histologic grade and serous papillary adenocarcinomas of the endometrium, it did not correlate with invasion, metastasis, or clinical stage. Comparison of the staining patterns with molecular analysis of mutations in the p53 gene showed the expected correlation of nuclear staining with missense mutations but not with nonsense mutations, which consistuted one third of all mutations found in this series. In addition, cytoplasmic staining did not predict mutation.