AbstractLoss of DNA mismatch repair occurs in a variety of malignancies and is associated with genome‐wide instability of microsatellite repeats, a molecular phenotype referred to as microsatellite instability (MSI). MSI is a consistent feature of colorectal and endometrial tumors from patients with hereditary non‐polyposis colorectal cancer (HNPCC). Sporadic colorectal and endometrial cancers that exhibit MSI frequently have methylation of the MLH1 promoter. We undertook a detailed family and medical history study to compare family cancer risk for women with MSI‐positive and ‐negative endometrial cancers. The MLH1 promoter methylation status was determined for all cancers. Family histories were developed for 80 probands (40 with MSI‐positive and 40 with MSI‐negative tumors). The numbers of reported cancers in first‐ and second‐degree relatives of the 2 groups were similar. There was a modest increase in familial cancer clustering for MSI‐positive probands. When MSI‐positive tumors were subclassified according to MLH1 promoter methylation, a clear association between methylation status and familial cancer risk was evident. Women with MSI‐positive endometrial cancers in which the MLH1 promoter was unmethylated had a 7‐fold relative risk (RR) of demonstrating familial clustering of cancers [RR 7.07 (95% confidence interval 2.29–21.81)]. The women with MSI‐positive, MLH1‐unmethylated tumors were significantly younger than the rest of the study population (56.1 years vs. 65.4, p ≤ 0.01). Age of onset and tumor MSI not associated with MLH1 promoter methylation may point to women with a genetic susceptibility to malignancies. © 2002 Wiley‐Liss, Inc.