Some low-grade endometrioid carcinomas arise from a background of endometrioid tumours of borderline malignancy. To determine the molecular mechanisms involved in the initiation of endometrioid carcinoma, the present study investigated whether the genetic alterations reported in these tumours (mutations in PTEN, KRAS, and beta-catenin genes, and microsatellite instability) are already present in endometrioid tumours of borderline malignancy. Eight endometrioid tumours of borderline malignancy were studied. By immunohistochemistry, beta-catenin was expressed in the nuclei of all tumours, suggesting the presence of stabilizing beta-catenin mutations. By mutational analysis, five different beta-catenin mutations were found in seven of eight cases (90%), affecting codons 32, 33, and 37. In contrast, only one tumour harboured a PTEN mutation, which affected codon 130. Neither KRAS mutations nor microsatellite instability was detected. A review of the literature indicated that beta-catenin mutations are characteristic of well-differentiated endometrioid carcinomas, since they were present in nearly 60% of grade I but in less of 3% of grade III tumours. In conclusion, the present study identifies beta-catenin mutation as a nearly constant molecular alteration in borderline endometrioid tumours, whereas PTEN and KRAS mutations and microsatellite instability are very infrequent. The findings in the present study, and previously reported data, strongly suggest that beta-catenin mutation is an early event in endometrioid ovarian carcinogenesis, and that it is involved in the development of low-grade endometrioid tumours.