The behavior of preschool children from five groups (developmental language disordered, high-functioning autistic, low-functioning autistic, mentally retarded, and normally developing) were coded in three situations: presentation of a nonsocial orienting stimulus (an unfamiliar noise) and two social situations involving simulated distress on the part of an adult with whom they were playing. Cognitive level was correlated with level of responsiveness to stimuli only for the two retarded groups (mentally retarded and low-functioning autistic). Girls showed more prosocial behavior than boys in both social situations, independent of diagnosis. The language-disordered children showed only mild and subtle social deficits. The low-functioning autistic children showed pronounced deficits in responding in all situations. The mentally retarded and high-functioning autistic children showed good awareness of all situations, but were moderately impaired in their ability to respond prosocially; they rarely initiated prosocial behavior, but did respond to specific prompts. The behavioral feature that marked both autistic groups, in contrast to all other groups, was a lack of social referencing; they did not tend to look toward an adult in the presence of an ambiguous and unfamiliar stimulus. Results are discussed in terms of variability between and among high- and low-functioning autistic children, and implications for the core deficits in autism.