The current study examined predictors of treatment use among 56 foster mothers who participated in an attachment-based intervention program for foster infants. Foster mothers' levels of treatment use were coded at early, middle, and late phases of the intervention program. Foster mothers' states of mind with regard to attachment predicted their understanding of the intervention session concepts. Specifically, autonomous foster mothers showed higher levels of understanding at the start of the intervention program, when compared with non-autonomous foster mothers. State of mind with regard to attachment also predicted foster mothers' levels of reflective functioning during the intervention sessions. Autonomous foster mothers showed higher levels of reflective functioning at early, middle, and late stages of the intervention program, when compared with non-autonomous foster mothers. The relevance of these findings for both treatment effectiveness and treatment delivery is discussed.