Objective: Atypical antipsychotic medications have largely supplanted their typical counterparts, both for psychosis and for the treatment of aggression and/or self-injurious behaviour (SIB), in persons with intellectual disabilities (ID). However, with the exception of risperidone, little systematic research supports their use in such persons. Method: A retrospective review of 31 adult residents of a state developmental centre, who were treated for aggression and/or SIB with atypical antipsychotics. Average monthly counts of aggression and SIB for 1 year of treatment with typical antipsychotics, were compared with monthly averages for the next 12 months of treatment with atypical antipsychotics. Results: Twenty-seven of 31 subjects (87%) completed a full year of atypical antipsychotic treatment. Subjects ranged in age from 24 to 54 years (mean = 39); 1831 (58%) had profound ID. Twelve of 26 (46%) had typical antipsychotics discontinued within the year of atypical treatment; another 726 (27%) had their typical antipsychotic dose decreased. Twenty-three of 31 trials involved risperidone; 731 olanzapine; 131 quetiapine. Subjects gained an average of 6.6 pounds during the year of atypical treatment, but no significant changes in glucose or cholesterol were found. Subjects with aggression alone (N = 14) had significant decreases in the number of aggressive acts per month during the year of atypical treatment (P = 0.03); those with both aggression and self-injury (N = 12), or those with self-injury alone (N = 5) had no significant improvement. Conclusion: The findings suggest that atypical antipsychotics can be successfully substituted for typical agents in individuals with ID and decrease the frequency of aggression over one year of treatment. The weight gain seen in our sample reinforces the necessity of regular monitoring of weight and metabolic changes in persons with ID treated with atypical antipsychotics.