No study has analyzed the relative occurrence of a broad range of symptoms reported by mothers of children with autism, ADHD-Combined, and ADHD-Inattentive and typical controls. Mothers rated 1436 children with autism, 1056 with ADHD without autism, and 186 controls, 2-17 years, on 41 internalizing, externalizing, neurodevelopmental, and somatic problems. Most children with autism had symptoms of ADHD, oppositional defiant disorder, disruptive mood dysregulation disorder, and expressive language disorder and almost half had dysgraphia and receptive language disorder. Symptom overlap between autism and ADHD-Combined was high. Clinicians specializing in autism and ADHD must have expertise in evaluating and treating these comorbidities identified as most problematic by mothers in order to relieve family concerns and develop treatment plans relevant to families.