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A genealogical analysis and molecular and clinical investigations were carried out in 19 probands with multiple colorectal adenomas (numbering near 100 or more). Twelve of these patients (63.1%) were negative for the APC and MYH gene mutations using conventional methods. A hereditary form of the disorder was confirmed in three (25%) probands. The median age of the disease onset in APC-negative patients was somewhere between the mean age of the disease onset in APC- and MYH-positive polyposis. Extraintestinal signs of the disorder were less frequent in APC-negative than in APC-positive probands. Half of APC- and MYH-negative probands with multiple polyposis developed colon cancer. APC- and MYH-negative patients formed a genetically heterogeneous group. [ABSTRACT FROM AUTHOR]