Twenty-six adolescents with a chronic mental illness (schizophrenia or schizoaffective disorder of at least 1.5 years' duration) were assessed for the presence or absence of comorbid substance abuse. The two groups were compared on a number of variables believed to identify or predict substance abuse. The substance abusing subgroup were significantly different in levels of social functioning, school achievement, premorbid substance abuse, having parents or siblings who abused substances, dysfunctional families, cigarette smoking, number of hospital admissions, and emergency room visits. These findings are discussed in the context of clinical issues regarding the management of adolescents with chronic mental illnesses.