Background: Social and emotional adjustment of siblings of children with autism was examined, to explore their risk or resilience to effects of genetic liability and environmental factors involved in having a sibling with autism. Method: Social-emotional adjustment, behavior problems, socialization skills, and siblings' relationships were compared among 30 siblings of children with autism, 28 siblings of children with mental retardation of unknown genetic etiology (MR), and 30 siblings of children with developmental language disorders (DLD). Groups were matched by probands' gender, siblings' chronological age, gender, IQ, and birth order, and by family size, ethnicity, and parental income, employment, and stress level. Results: Four siblings of children with autism, three siblings of children with MR, and seven siblings of children with DLD received DSM-IV diagnoses. Nevertheless, most of the siblings were well adjusted. Conclusions: The adjustment of siblings of children with autism is in sharp contrast to the severe social and emotional disabilities characteristic of autism, and is noteworthy considering the stress involved in having a sibling with autism.