Familial adenomatous polyposis (FAP) in a parent requires diagnostic follow-up and treatment from adolescence in possible gene carriers in order to prevent cancer development. A nationwide sample (n = 22) of adolescent FAP offspring including 85% of eligible individuals aged 11-20 years and their parents were interviewed with regard to adolescent mental health, psychosocial functioning, knowledge about FAP and genetic risk, and experiences with testing and surgery. Thirty-six percent of the FAP offspring fulfilled criteria for a psychiatric diagnosis. For adolescents older than 15 years, this was increased relative to a comparison group with Hirschprung’s disease and a general population sample. Neither genetic testing nor FAP diagnosis in adolescent FAP-offspring differentiated significantly between those who fulfilled the criteria and those who did not for psychiatric diagnosis, while a global score of chronic family difficulties did. This may imply that experiencing parental illness more than inheriting FAP is a perceived stressor for adolescent FAP offspring.