The Short Sensory Profile was used to assess parental report of sensory reactivity across four groups of young children (n = 102). Groups were autism (n = 26), fragile X syndrome (n = 20), developmental disabilities of mixed etiology (n = 32), and typically developing children (n = 24). Groups were comparable on overall mental age (x = 22 months), and clinical groups were comparable on chronological age (x = 31 months). Significant differences were detected at alpha <.01 for tactile sensitivity [F(3,99) = 10.01], taste/smell sensitivity [F(3,99) = 11.63], underreactive/seeks stimulation [F(3,99) = 4.56], auditory filtering [F(3,99) = 19.67], and low energy/weak muscles [F(3,99) = 14.21]. Both children with fragile X syndrome and children with autism had significantly more sensory symptoms overall than the two comparison groups, and children with autism did not differ significantly from children with fragile X syndrome. Both groups were more impaired than developmentally delayed and typically developing children in tactile sensitivity and auditory filtering. Children with autism were more abnormal in responses to taste and smell than all other groups. Children with fragile X syndrome were more abnormal than all other groups in low energy/weak muscles. Sensory reactivity of children with developmental delays was comparable to mental age-matched typically developing toddlers. Correlational analyses indicated that neither overall developmental level nor IQ was related to abnormal sensory reactivity in children with autism or general developmental disorders. However, abnormal sensory reactivity had a significant relationship with overall adaptive behavior.