OUTLINE: Participants complete a baseline assessment of knowledge, risk perception, and personality traits followed by a structured pretest education session. Participants are then offered the choice of whether or not to undergo genetic testing for mutations in hereditary nonpolyposis colon cancer (HNPCC) genes. Participants who choose to undergo genetic testing provide a blood sample for this purpose. Participants who undergo genetic testing receive results and counseling (recruitment of new families with HNPCC stopped as of 04-26-06, recruitment of persons within families already participating continues).