Background: Research on addiction treatment utilization in indigent samples mainly has been retrospective, without measures of addictive consequences, social network influences, and motivation. Prospective assessment of factors influencing utilization could inform policy and clinical care. Objective: We sought to identify factors associated with utilization of addiction treatment and mutual help groups among substancedependent persons with high rates of homelessness. Research and Methods: This was a prospective cohort of patients detoxified from alcohol or drugs at baseline who were followed for 2 years in a randomized clinical trial of linkage to primary care (n = 274). Outcomes included utilization of Inpatient/Residential, Outpatient, Any Treatment, and Mutual Help Groups. Predictor variables in longitudinal regression analyses came from the literature and clinical experience, organized according to theoretical categories of Need, and non-Need (eg, Predisposing and Enabling). Results: Many subjects used Inpatient/Residential (72%), Outpatient (62%), Any Treatment (88%) or Mutual Help Groups (93%) at least once. In multivariable analyses, addictive consequences (odds ratio [OR] 1.38, 95% confidence interval [CI] 1.12-1.71), motivation (OR 1.32, 95% CI 1.09-1.60), and female gender (OR 1.80, 95% CI 1.13-2.86) were associated with most treatment types (ORs are for Any Treatment). Homelessness was associated with Residential/Inpatient (for Chronically Homeless vs. Housed, OR 1.75, 95% CI 1.04-2.94). Living with one's children (OR 0.51, 95% CI 0.31-0.84) and substance-abusing social environment (OR 0.65, 95% CI 0.43-0.98) were negatively associated with Any Treatment. Conclusions: In this cohort of substance-dependent persons, addictive consequences, social network variables, and motivation were associated with treatment utilization. Non-need factors, including living with one's children and gender, also were significant. Copyright © 2006 by Lippincott Williams & Wilkins.