BACKGROUND: In patients with colorectal carcinoma, insertions or deletions of short sequences of DNA, a phenomenon called microsatellite instability, are observed. AIM: To look for microsatellite instability and mutations of MLH1 and MSH2 gene mutations in patients with colorectal carcinoma. MATERIAL AND METHODS: Ten patients with sporadic colorectal carcinoma and 31 patients fulfilling criteria for hereditary nonpolyposis colon cancer (HNPCC), aged 9 to 70 years, were studied. Microsatellite instability was studied in