Catatonia is a life-threatening disorder characterized by motor abnormalities, mutism, and disturbances of behaviour, which is increasingly being diagnosed in persons with autism. In this report, we describe the presentation and course of catatonia in an adolescent with autism who responded to electroconvulsive therapy (ECT). The illness started with depressive symptoms, but the predominant feature was one of extreme obsessive slowing and immobility. We propose that catatonia should be ruled out as a cause of regression sometimes seen in adolescents with autism, and that catatonia of autism may index a distinct subtype with a particularly poor outcome.