The current study aimed to determine whether cocaine use compromises treatment outcomes for opiate users. Data were collected from 404 opiate users at treatment intake and 1-year follow-up as part of a national treatment outcome study. Because of higher intake measures, cocaine users improved in more outcomes than nonusers, but comparisons between groups found that cocaine users had more coexisting problems. Regression analysis revealed that those who used cocaine at intake were more likely to use cocaine at 1-year follow-up, to commit crime, and to be homeless. It is concluded that treatment for opiate use “works” even in the presence of concurrent cocaine use.