This study investigated whether adolescent's family type was associated with regular smoking or the use of illicit substances (cannabis or hard drugs) among underage adolescent psychiatric in-patients. The sample consisted of 471 adolescents aged 12-17 years admitted to psychiatric hospital between April 2001 and March 2006 at Oulu University Hospital, Finland. The information on family factors and substance use was based on the Schedule for Affective Disorder and Schizophrenia for School-Age Children, Present and Lifetime interview and the European modification of the Addiction Severity Index questionnaire. Compared to adolescent boys from two-parent families, those from child welfare placement were more likely to regularly use both cannabis (odds ratio [OR]=4.4; 95%confidence interval [CI]=1.4-13.7; P=.012) and hard drugs (OR=8.4; 95% CI=1.7-42.1; P=.01).Among girls, no association was found between family type and the use of illicit substances. Two-parent or foster family units may protect adolescents from involvement with illicit substances. In clinical adolescent psychiatric practice more attention should be paid to family interventions and parental support.