This study evaluated the effects of short-term manualized autism spectrum disorder (ASD)-modified "interaction therapy" (IT) for 42 parents, comprising both fathers and mothers of 21 preschoolers with ASD, while comparing father and mother outcomes. Intervention features included parents' focus on self-reflection and "here-and-now" investigation of live parent-child interaction, both parents' equal participation, spousal observation, and nonhierarchical parent-therapist collaboration. Pretest and posttest data included analysis of videotaped father-child and mother-child interactions to assess parental responsiveness and affect, and questionnaires on parents' perceived attachment to child. Results following ASD-modified IT showed both parents' increases in responsiveness and affect during parent-child interaction, with a larger increase in mothers' affect. No change emerged in father's perceived attachment, but mothers increased their perceived attachment quality and pleasure from interaction. Findings extended prior research, which had mostly examined mothers' improvements following caregiver-mediated intervention, by pinpointing the ASD-modified IT's efficacy for promoting not only mothers' but also fathers' responsive and affective interaction abilities.