Parents of children with autism in applied behavior analytic treatment were surveyed on their use and perceptions of supplemental treatments. In study 1, respondents (N=121) reported enrolling their children in an average of seven supplemental interventions, most commonly non‐behavior‐analytic speech therapies (85%), megavitamins (61%), Sensory Integration Therapy (56%), and elimination diets (50%). In study 2, parents (N=24) were interviewed about these four interventions and typically reported little or no benefit. Factors that parents cited as influencing their decision to obtain supplemental interventions for their children varied substantially, depending on the individual intervention. Because of the extensive use of unvalidated interventions that parents deemed unhelpful, it seems important to reduce children's participation in such interventions. However, because of the diverse influences on parents' decisions to obtain the interventions, a multifaceted strategy may be needed. Copyright © 2000 John Wiley & Sons, Ltd.