Using both self-determination theory (Ryan & Deci, 2000) and Dweck's (1991) entity versus incremental personality distinction, we examined the role of motivation as a predictor of treatment success in a methadone maintenance program. Specifically, it was predicted that internal motivation and perceived autonomy support would be associated with better treatment adherence as indicated by negative urine tests, attendance, and the attainment of take-home methadone dosages, whereas external motivation was not expected to enhance these outcomes. Results generally supported these hypotheses, yet also indicated that high levels of external motivation coupled with low levels of internal motivation predicted particularly poor treatment outcomes. In addition, patients embracing an entity belief that their addiction was a fixed aspect of self also attained better outcomes. Results are discussed in terms of the dynamics of motivation in addiction treatments.