Background At least five mismatch repair (MMR) genes, including hMSH2, hMLH1, hPMS, hPMS2, and hMSH6/GTBP, are associated with hereditary nonpolyposis colorectal cancer (HNPCC). More than 90% of families with HNPCC harbor the hMSH2 and hMLH1 gene mutations. We have analyzed the clinical features of HNPCC among Chinese patients and report the results of screening for mutations in the hMSH2 and hMLH1 genes.Methods The data concing gender, site of colorectal cancer (CRC), age at diagnosis, history of synchronous and/or metachronous colorectal cancer, instance of extracolonic cancers, and histopathology of tumors for 126 patients from 28 independent families with HNPCC were collected. Fifteen of the families met the Amsterdam I criteria, and 13 met the Japanese clinical criteria for diagnosis. Genomic DNA was extracted from the peripheral lymphocytes. Polymerase chain reaction (PCR) and denaturing high-performance liquid chromatography (DHPLC) were used to screen the coding region of the hMSH2 and hMLH1 genes. Samples showing abnormal DHPLC profiles were sequenced.Results One hundred and seventy malignant neoplasms were found in the 126 patients, of whom 23 had multiple cancers. Ninety-eight of the patients (77.8%) had colorectal cancers, with an average age at onset of 45.9 years and a right-sided predominance. Eight hMSH2 or hMLH1 gene sequence variations were found in 12 families, and a germ-line G204X nonsense mutation in the third exon of hMSH2 was found, representing the first mutation in an MMR gene ever found in people of Chinese Mongolian ethnicity.Conclusions HNPCC is a typical autosomally dominant hereditary disease, characterized by early onset, a predominance of proximal colorectal cancer, and multiple synchronous and metachronous colorectal cancers. DHPLC is a powerful tool for detecting mutations in the hMSH2 and hMLH1 genes. Mutations in the first nine exons of the hMLH1 gene were more common in Chinese patients.