This pilot study examined the efficacy and tolerability of olanzapine in the treatment of children, adolescents, and adults with pervasive developmental disorders (PDDs). Eight patients with principal diagnoses (DSM-IV) of autistic disorder (N = 5) or PDD not otherwise specified (N = 3) were given olanzapine in an open-label, prospective fashion for 12 weeks. Clinical ratings were obtained at baseline and at the end of weeks (EOWs) 4, 8, and 12. Seven of eight patients completed the 12-week trial, and six of the completers were deemed clinical responders as measured by ratings at the EOW 12 of "much improved" or "very much improved" on the global improvement item of the Clinical Global Impression Scale. Significant improvements in overall symptoms of autism, motor restlessness or hyperactivity, social relatedness, affectual reactions, sensory responses, language usage, self-injurious behavior, aggression, irritability or anger, anxiety, and depression were observed. Significant changes in repetitive behaviors were not observed for the group. The EOW 12 mean +/- SD daily dose of olanzapine was 7.8 +/- 4.7 mg/day. The drug was well tolerated with the most significant adverse effects noted to be increased appetite and weight gain in six patients and sedation in three. With respect to weight gain, the mean +/- SD weight for the group increased from 137.50 +/- 55.81 pounds (62.50 +/- 25.37 kilograms) at baseline to 155.94 +/- 55.13 pounds (70.88 +/- 25.06 kilograms) at EOW 12. No evidence of extrapyramidal side effects or liver function abnormalities was seen. These preliminary results suggest that olanzapine may be an effective and well tolerated drug in targeting core and related symptoms of PDDs in children, adolescents, and adults. Further studies, particularly those that are placebo-controlled and double-blinded, are indicated to better define the clinical use of olanzapine in these patient populations.