Little research has been conducted on whether deficits in developmental functioning affect the range of core symptoms for autism spectrum disorders (ASD). This study represents a first attempt to determine whether developmental level has an effect on the expression of ASD symptoms in infants and toddlers. Eight hundred and fifty-three infants were evaluated with respect to the nature and extent of their ASD symptoms and developmental functioning. Young children with autism displayed a higher number of symptoms than those with PDD-NOS on all three domains of impairment (social, communication, repetitive behaviors). As expected, children without an ASD evinced far fewer symptoms than both these groups. Developmental level was not found to be a moderator for expression of ASD symptoms for the entire sample, or individual diagnostic groups. Higher developmental level was associated with lower severity of evinced ASD symptoms in the sample. (Contains 1 table and 3 figures.)