We examined the impact of father involvement on treatment. Participants were 107 families enrolled in parent-child interaction therapy (PCIT), including 56 involved-father (IF) families, 16 uninvolved-father (UF) families, and 35 absent-father (AF) families. All groups showed improvements during treatment to within the average range on the Eyberg Child Behavior Inventory (ECBI), although mothers from AF families reported better treatment outcome than mothers from IF families. Improvements occurred on the Beck Depression Inventory (BDI) and the Parenting Stress Index (PSI) as well, but there were no group differences. At a 4-month follow-up, mothers in IF families maintained treatment gains on the ECBI. In contrast, mothers in AF families reported significant decline at follow-up, although their scores remained within the normal range. Results suggest that father participation in treatment may not affect immediate treatment outcome but may help to maintain the beneficial effects of PCIT.