Recently, a high rate of endometrial cancer has been reported in women with hereditary non‐polyposis colorectal cancer (HNPCC), suggesting a relationship between familial endometrial cancers and HNPCC. Familial endometrial cancers constitute only about 0.5% of all endometrial carcinomas and it is essential to examine family histories in detail. A mutational analysis of three DNA mismatch repair (MMR) genes (hMLH1, hMSH2 and hMSH6) in patients with endometrial cancer who meet our criteria for familial predisposition to HNPCC‐associated endometrial cancers was performed. Mutations were detected in 18 of the 120 patients (15.0%). Most HNPCC‐related endometrial cancers do not meet the New Amsterdam Criteria for HNPCC. These clinical criteria may identify only some HNPCC‐associated endometrial cancers. Establishing the correct family history for endometrial cancer patients is important for diagnosing familial endometrial carcinomas. An analysis of MMR genes may be useful for patients with endometrial cancer showing familial aggregation. In addition, gynecologists must be accurately informed, and it is important to perform large‐scale, multicenter studies both nationwide and internationally. (Cancer Sci 2008; 99: 1715–1719)