A substantial proportion of pathological gamblers engage in gambling-related illegal behavior. We examined differences in baseline characteristics and treatment outcomes in two groups: pathological gamblers who did and did not commit gambling-related illegal acts in the year before treatment. Participants were 231 pathological gamblers enrolled in a randomized study of treatment that included cognitive behavior therapy and referral to Gamblers Anonymous (GA). Participants reporting recent illegal behavior (n = 63) endorsed more severe lifetime and recent (past-year) gambling disorder symptoms and higher gambling-related debt than did gamblers who denied illegal behavior (n = 168). Those who reported illegal behavior also maintained a significantly higher severity of gambling disorder throughout treatment, although both groups experienced similar improvements in gambling symptoms over time. While pathological gamblers with or without gambling-related illegal behavior appeared to improve at a similar rate regardless of the treatment provided, more intensive treatment may be warranted for individuals with gambling-related illegal behavior, as they demonstrated greater gambling severity throughout treatment and follow-up.