This article uses Markov analysis to investigate patterns of treatment participation of 361 patients treated in the alcohol and drug abuse programs of a large group model Health Maintenance Organization (HMO) to examine how participation is related to abstinence. Findings indicate that 82% of the patients in treatment one month after intake were in treatment three months later, and treatment retention dropped to 46% by month 6. Findings also indicate that 74% of patients abstinent and in treatment at month 1 remained so at month 3. Abstinence at the first three-month interval was a strong predictor of abstinence at later time periods. A multivariate analysis showed that an expressed desire to stop alcohol use upon entry into treatment was the most consistent predictor of both treatment participation and abstinence at most time points. Treatment participation was also a significant predictor of abstinence.