Hereditary non-polyposis colorectal cancer (HNPCC or Lynch syndrome) is characterized by the early onset of colorectal neoplasms, frequently localized in the right colon, increased occurrence of multiple primaries, vertical transmission and aggregation of tumours in families in accordance to a Mendelian dominant type of inheritance. The syndrome accounts for approximately 5% of all colorectal cancers. The purpose of the present study was to describe the tumour spectrum and the most relevant clinical features of 28 kindreds with HNPCC, classified according to the guidelines of the International Collaborative Study Group, and of 61 "suspected" HNPCC. These families were observed during a 6-year registration of colorectal neoplasms in a health-care district of Northern Italy. Colorectal cancer was by far the most frequent malignancy; gastric cancer was the second. Uterine carcinoma was only slightly more frequent than expected. Lung- and breast-tumour rates were lower than expected. Cancer distribution in the large bowel showed that about two fifths of the tumours developed in the right colon. The occurrence of cancer before the age of 50 to 60 was much more frequent in HNPCC. Multiple tumours developed in 25 patients with HNPCC and in 32 with "suspected" HNPCC. Pancolonoscopy remains the procedure of choice for surveillance; other examinations, such as gastroscopy, gynaecological investigations, urography and cholangiography, are suggested only to selected families. One of the main features of the study was the inclusion of 61 "suspected" HNPCC, a heterogeneous group of families which nonetheless deserves careful follow-up.