AIM: To descr ibe systematically the clinical characteristics and phenotype of HNPCC families and the prevalence of HNPCC in the general population of CRC patients in China.METHODS: HNPCC kindreds and CRC patients were from two sources. One was that we consecutively investigated kindreds and patients by ourselves. And the other was the published Chinese and foreign literature related to Chinese HNPCC syndrome. There were 142 HNPCC families fulfilling AC Ⅰ and/or AC Ⅱ including 57 families with detailed data, and 3874 general primary CRC patients in all. All statistical tests were two-sided.RESULTS: In AC Ⅰ families, the number of Lynch syndrome Ⅰ and Ⅱ families were 25 (47.2%) and 28 (52.8%)respectively. There were 215 patients (82.4%) with CRC,67 patients (25.7%) with extracolonic cancer and 50patients (19.2%) with multiple primary cancers. In all CRC patients, multiple primary CRC were in 41 patients (19.1%),and the first-CRC was right-sided colorectal cancer in 143 patients (66.5%) and rectal cancer in 44 patients (20.5%). 8.8% and 19.2% of the first cancer were CRC and extracolonic cancers. Among those patients whose first cancer was CRC, 66.8% and 19.9% were right-sided colorectal cancer and rectal cancer, respectively. The similar results were found in AC Ⅱ families. Normal distribution was only found in the distribution of the age of diagnosis of the first cancer in both AC Ⅰ families (coefficient of skewness: u = 0.81, 0.20＜0.40＜P＜0.50;coefficient of kurtosis: u = 1.13, 0.20＜P＜0.40, α = 0.20)and AC Ⅱ families (coefficient of skewness: u = 0.63, P＞0.5＞0.20; coefficient of kurtosis: u = 0.84, 0.20＜0.40＜P＜0.50,α = 0.20), but not found in the distribution of the age of diagnosis of the first CRC. When patients with HNPCC-associated cancer suffered from the first malignant tumor in HNPCC families diagnosed by AC Ⅰ and AC Ⅱ, the mean age and median age were 45.1±12.7 years and 44.0 years,45.2±12.7 years and 44.5 years, respectively. The median age of diagnosis of the first tumor of the patients in the later generation was younger than that in the previous generation. Many extracolonic cancers were found to be associated with HNPCC syndrome. Gastric cancer was the most frequent extracolonic cancer followed by endometrial cancer and hepatocarcinoma. In general population of CRC patients, the prevalence of HNPCC diagnosed by AC Ⅰ and AC Ⅱ were 1.3% and 2.2%, respectively.CONCLUSION: The clinical phenotype and prevalence of Chinese HNPCC syndrome are similar to those of Europeans and Americans. Gastric cancer is the most common extracolonic malignant tumor. The age of diagnosis of the first malignant tumor tends to be increasingly younger in patients with HNPCC-related tumors.