Outcome predictors between two methadone maintenance treatment clinics in Tel-Aviv, Israel, and Las Vegas, Nevada, were determined by comparing patients' characteristics. All patients admitted to the two clinics (302 from Las Vegas and 492 from Tel-Aviv) were studied with respect to variables at admission and follow-up. Las Vegas patients were older, contained more females, had more hepatitis C positive markers, and more urine analyses that were positive for cocaine, amphetamines, and tetrahydrocannabinol (THC) on admission than the Tel-Aviv patients. After 1 year, Tel-Aviv patients had higher retention (73.6% vs. 61.6%) and similar opiate abstinence (65.8% vs. 64.9%) compared to Las Vegas patients. Predictors for cumulative retention (Cox regression) for both clinics were higher methadone dosages greater than or equal to 100 mg/day (Tel-Aviv OR [odds ratio] = 2.1, 95% confidence interval [CI] = 1.6-2.9; Las Vegas OR = 1.8, 95% CI = 1.3-2.5). Also, in Tel-Aviv, predictors were no opiate use after 1 year (OR = 1.7, 95% CI = 1.4-2.2) and no benzodiazepine after 1 year, and in Las Vegas no cocaine and no amphetamines after 1 year and age less than or equal to 30 years. The two major predictors in the two clinics were successful in both outcomes: 1 year retention and opiate abstinence.