Expressed emotion measures the emotional climate of the family and is predictive of symptom levels in a range of medical and psychiatric conditions. This study extends the investigation of the effects of expressed emotion to families of individuals with autism. A sample of 149 mothers co-residing with their adolescent or adult child with autism over an 18-month period was drawn from a large multiwave longitudinal study. High expressed emotion was related to increased levels of maladaptive behavior and more severe symptoms of autism over time. Also, characteristics of the son or daughter influenced levels of maternal expressed emotion over time. Although autism is a complex genetic disorder, the effect of the family environment in shaping the behavioral phenotype should not be underestimated.