We sought to determine the prevalence, patterns, and correlates of past-month illicit methadone use and history of regular illicit use among stimulant-using methadone maintenance treatment patients. We obtained self-reported information on illicit methadone use from 383 participants recruited from six community-based methadone maintenance programs. Overall, 1.6% of participants reported illicit use in the past month, and 4.7% reported a history of regular use. Younger age and history of outpatient psychological treatment were associated with increased odds of past-month illicit use. Illicit methadone use among patients in maintenance programs is infrequent; however, a number of factors may increase risk of illicit use.