p16MTS1/CDKN1 and the retinoblastoma protein Rb are both involved in negative regulation of G1/S progression in the mammalian cell cycle. Inactivation of one of these tumour suppressor genes is involved in many malignant tumours, and in some studies a negative correlation of p16 and Rb expression has been found. In order to study this interaction in endometrial carcinogenesis, we investigated 36 endometrial carcinomas, 11 cases of hyperplasia, 23 normal endometrial samples, and two uterine carcinoma cell lines by immunohistochemistry or RT-PCR. Rb was expressed in normal endometrial epithelium, hyperplasia, cell lines, and most carcinomas; negative immunostaining was only detected in 1 of 36 tumours. In contrast, p16 expression was weak in normal endometrium and increased in most cases of hyperplasia, but negative or minimally positive in 74% of the carcinomas and the Hec1B adenocarcinoma cell line, and there was no significant association with Rb immunostaining. Strikingly high p16 expression was found in foci of squamous metaplasia within hyperplastic or carcinomatous tissue. Deletion and mutation analysis of the p16 gene was performed in DNA from microdissected tumour samples and cell lines. No p16 deletion was found, and mutations were detected in only one tumour sample and Skut1B uterine mixed mesodermal tumour cells. Our data indicate that in spite of low or absent p16 expression, genetic alterations of the p16 and Rb tumour suppressor genes are rare in endometrial carcinogenesis.