Abstract Programs for drug abusers are plagued by high rates of dropout. Because of the strong relationship between longer treatment and positive outcome, researchers have begun to study individual and program-specific factors that influence premature termination of treatment. For the most part, these studies have focused on dichotomous measures of dropout or number of sessions attended. In this article, we extend this line of research in two ways. First, we develop and measure a number of indicators of treatment participation based on therapist ratings. Second, we develop a model of treatment participation that employs both individual and program-specific factors. The data show that tremendous variation in participation occurred even among those who attended a majority of sessions, which highlights the importance of obtaining more elaborate measures of treatment participation. The model predicting treatment participation suggests that initiation of heroin use later in life, continued use of marijuana, and behavioral indicators of motivation are the strongest predictors of program participation. Research and practical implications of the findings are discussed.