In this report we present data on cognitive development, language, behavior and social skills in 12 children and adolescents, nine girls and three boys, aged between 2.5 and 19 years, with Coffin-Siris syndrome (CS). 1. Mental retardation was mild in three patients and moderate in the nine others. 2. Speech onset was severely retarded with little interest in language. In the older group (seven patients aged 7 to 19 years), language comprehension was appropriate to the mental level. 3. Gross-motor functioning and autonomy, with the lowest score on ''Task-orientation'', were equal to the mental development. 4. Most frequently, aggressive disturbed behavior was observed, especially in the youngest children, while mixed disturbed behavior was observed in the oldest patients. Almost half of the CS patients (5/11) presented symptoms of pervasive developmental disorder, with 2/11 scoring in the pathological range. Obsessive interests, strong dependence on patterns and rituals and unusual fears were characteristic behavioral problems also when they got older.