In general, people with dual diagnoses account for a significant proportion of both the mental health and substance abuse populations. Most published information on dual diagnosis comes from research on selected treatment programs that are largely funded from public sources. This analysis uses private health insurance claims and eligibility files for 1989 to 1991 for three large firms to identify individuals with both substance abuse and mental health claims and to examine their characteristics, charges, and utilization. More than half of people with dual diagnoses incurred significant charges over three years in both mental health and substance abuse. These individuals with high mental health charges were more likely to be male than were patients with mental health claims alone; they were less likely to be male than were patients with claims for substance abuse and no mental health services. They were also significantly younger than were patients with substance abuse or mental health utilization only for two of the firms. The average charges for people with dual diagnoses were higher than those for patients with substance abuse or mental health claims only.