The study examined the spontaneous expressions of affect displayed by 18 autistic and 18 mentally retarded children matched for chronological age and mental age, as well as a group of 18 normal children matched for mental age only. Affect expressions were coded from videotapes of a standard child-experimenter interaction designed to assess prelinguistic communication skills. The experimenter presented the child with different toys, initiated social games and turn-taking activities, pointed at posters around the room, and made simple requests of the child. Results indicated that the autistic subjects were not more neutral or flat in their affect compared to the mentally retarded and normal subjects. However, the autistic children showed a greater variety of affect expressions and spent more time displaying discrete negative affect expressions. In particular, they displayed negative and incongruous blends not displayed by any of the other children. This unique pattern of autistic children's affect expressions may be syndrome-specific, and may contribute to the difficulty that others experience in reading the affective signals of autistic children. (Author/JW)