Reiss et al. (1985) described two autistic children with the Williams syndrome, a dysmorphic developmental syndrome of unknown cause. Both children also showed elevated blood serotonin levels. The present report describes two prepubescent females with the characteristic features of Williams syndrome, who are not autistic and who have blood serotonin levels within the normal range. These findings suggest that further study of developmental disorders that coexist with autism may help clarify the relationship between autism and putative biological markers such as hyperserotonemia.