Hereditary non-polyposis colorectal cancer (HNPCC), also called Lynch syndrome, is an autosomal dominantly inherited disorder of cancer susceptibility. Patients with HNPCC exhibit an increased risk for HNPCC-associated extracolonic tumors such as cancer of the endometrium. HNPCC is associated with germline mutations in DNA mismatch repair (MMR) genes: hMLH1, hMSH2 and hMSH6. Here, we describe two Japanese kindreds (0.5%) who met the new clinical criteria for HNPCC, Amsterdam criteria II, from among 375 endometrial cancer patients treated at Keio University Hospital from 1990 to 2002. From these results, it was found that female HNPCC patients comprised approximately 0.5% of all endometrial cancer patients. Decreased expression of two MMR gene protein products (hMLH1 and hMSH6) was confirmed immunohistochemically in these two endometrial tumors in HNPCC kindreds. This case report provides important information on Japanese HNPCC patients occurring endometrial cancer.