Background: Rates of participation in intervention research have not been extensively studied within autism spectrum disorder. Such research is important given the benefit of early intervention on long-term prognosis for children with autism spectrum disorder. The goals of this study were to examine how family demographic factors predicted treatment attendance and adherence in a caregiver-mediated randomized controlled trial targeting core deficits of autism spectrum disorder, and whether treatment attendance and adherence predicted outcome. Methods: In all, 147 caregiver-child dyads from a low-resourced population were randomized to in-home caregiver-mediated module or group-based caregiver education module treatment. Treatment attendance, adherence, and outcome (time spent in joint engagement) were the primary outcome variables. Results: The majority of families who entered treatment (N = 87) maintained good attendance. Attendance was significantly predicted by socioeconomic status, site, and treatment condition. Families in caregiver-mediated module reported lower levels of treatment adherence, which was significantly predicted by site, condition, caregiver stress, and child nonverbal intelligence quotient. Dyads in caregiver-mediated module had significantly longer interactions of joint engagement, which was significantly predicted by an interaction between treatment attendance and condition. Conclusions: Overall, the results from this study stress the importance of considering demographic variables in research design when considering barriers to treatment attendance and adherence.