Our study examined stability of self-regulation and affective expression in children with autism or Down syndrome over a 2 year period. A behaviorally anchored rating scale was used to assess a self-regulation factor (attention, adaptability, object orientation, and persistence), negative affect factor (hostility, irritability, and compliance), and positive affect factor (positive affect, affective sharing, and dull affect) from videotapes of play sessions involving each child and his or her mother. The patterns of ratings within each group were similar from time 1 to time 2, with the autism group showing more deviant ratings on measures of self regulation and affective sharing. From time 1 to time 2, children with autism showed relatively high stability for the self-regulation factor, but less stability than children with Down syndrome for all three factors.