The treatment and follow-up strategies of patients with hereditary nonpolyposis colorectal carcinoma (HNPCC) were analyzed in 22 Finnish HNPCC families in systematic follow-up between 1983 and 1990. During the seven-year study period metachronous colorectal neoplasia was diagnosed in 41 percent (15/37) of the patients treated by segmental colonic resection and in 24 percent (4/17) of those treated by subtotal colectomy. Extracolonic carcinoma was diagnosed in 12 (30 percent) of the 40 patients during the long-term follow-up. The most common extracolonic malignancy was biliopancreatic carcinoma which accounted for all five cancer-related deaths in the whole series during the study period. It was concluded that subtotal colectomy is superior to hemicolectomy or segmental resection in HNPCC patients with colorectal carcinoma. A regular annual endoscopic follow-up of the residual rectum is still necessary, and surveillance for extracolonic cancers must be considered.