OBJECTIVES: This study examined organizational trends from 1990 to 2000 and unit characteristics associated with the duration of nonmethadone outpatient addiction treatment. METHODS: Program directors and clinical supervisors from a nationally representative panel of nonmethadone outpatient units in the United States were surveyed in 1990, 1995, and 2000. Treatment duration was measured from clinical supervisors\u27 reports of the average length of stay. Negative binominal regression models controlled for multivariate effects. RESULTS: Treatment duration modestly declined between 1990 and 2000 while addiction severity increased. Affiliation with a mental health center, older program age, JCAHO accreditation-ostensibly a marker for structural quality-and serving more clients with prior authorization requirements- a measure of managed care stringency-were associated with shorter treatment durations. CONCLUSIONS: These findings suggest that treatment duration did not increase between 1990 and 2000 despite clients\u27 worsening addiction severity and growing evidence that longer duration of formal treatment improves treatment outcome. In addition, programs with JCAHO accreditation and stronger managed care oversight appeared to seek efficiencies through reductions in treatment duration