Individuals with both mental illness and alcohol or drug use disorders present distinctive treatment and human service problems, including increased risk for psychiatric hospitalization in community hospitals. Using national hospital discharge abstract data for 1990 and 1994, this study compared differences in psychiatric hospitalization in community hospitals of patients with mental illness only and those with mental disorders and substance use disorders. Individuals with dual diagnoses were younger, and a greater proportion were men. Medicaid was the primary payer for a larger percentage of those with dual diagnoses. Nationally, the number of commmunity hospitalizations for dually diagnosed patients increased 15 percent from 1990 to 1994, and total hospital charges increased from $1.9 to $2.2 billion.