The purpose of our study was to determine differences in autism severity and symptoms as a function of IQ, age, SES, gender, and race while simultaneously controlling these variables in 777 children with autism using a comprehensive measure evaluating 30 core and associated symptoms of autism. The children were 1-17 years of age with IQs from 9 to 146. Results showed that autism severity (total score on the Checklist for Autism Spectrum Disorder) and the 30 CASD item scores were not related to gender or race. However, the two CASD items reflecting mood and behavior problems were significantly more common in the lower than higher SES group. Our findings revealed significant, though modest, IQ and age effects. Autism severity increased with decreasing IQ and age, as did the frequency of 14 of the 30 CASD symptoms. The direction of IQ and age effects was the opposite for five symptoms and was nonsignificant for 11. Though IQ was associated with autistic symptoms, the majority of children with both high functioning and low functioning autism had most CASD symptoms and their symptom profiles were overwhelmingly similar. This supports the DSM-V Work Group's position that autism is a single spectrum disorder. (Contains 2 tables.)