Understanding of regression in autism has been hampered by variability in parental and clinical recognition and reporting of lost skills. This study introduced an instrument, the Regression Supplement Form, intended to supplement the Autism Diagnosis Interview-Revised and yield precise information about the types and timing of regression and events concurrent with loss and regain of skills. Data were collected from parents of 44 children (38 male, 6 female; mean age = 6 years) with Autistic Spectrum Disorder (37 Autistic Disorder, 7 Pervasive Developmental Disorder-Not Otherwise Specified). Parental responses on the Autism Diagnosis Interview-Revised indicated loss of skills during early development. The profile of regression that emerged included loss of skills between 18 and 21 months, on average, with language-only regression less common than loss of other, nonlanguage skills only or of full regression (loss of language and other skills). The onset of regression typically was gradual in nonlanguage areas and split between gradual and sudden loss for language skills. Some of the children were developing atypically before they lost other, nonlanguage skills, that is, their age at first words was delayed until age 2 years or older. Parents tended to attribute loss to medical factors such as immunizations. Many of the children regained some of the lost skills when they were 3.5-5 years of age, with therapeutic and instructional interventions given credit for the regain.