BACKGROUND Atypical antipsychotics may be useful in treating aggression associated with autistic spectrum disorder (ASD). We evaluated the effectiveness of low-dose quetiapine treatment in ASD adolescent patients with aggressive behavior. METHOD Eleven adolescent patients (8 boys and 3 girls) diagnosed with ASD, aged 13 to 17 years, were treated with quetiapine in an open-label study over an 8-week period. The severity of ASD, aggressive behavior, and sleep disturbances were assessed using the Clinical Global Impression-Severity (CGI-S), Overt Aggression Scale, and Child Sleep Habits Questionnaire, respectively. RESULTS Nonsignificant changes were obtained in autistic behavior after quetiapine treatment (CGI-S: 4.0 ± 0.6 vs CGI-S after: 3.1 ± 1.1; 2-tailed paired t = 1.93; df = 10; P = 0.08). Severity of aggressive behavior decreased significantly after quetiapine treatment (Overt Aggression Scale: 2.1 ± 0.94 vs 1.3 ± 0.64, respectively; 2-tailed paired t = 2.37; df =10; P = 0.028). Sleep disturbances improved significantly (Child Sleep Habits Questionnaire: 49.0 ± 12 vs 44.1 ± 9.6; 2-tailed paired t = 2.98; df =10; P = 0.014) and a positive correlation was found between the improvements in aggression and sleep (Spearman correlation: r = 0.43; N = 11; P = 0.013). Quetiapine was well tolerated. CONCLUSION Short-term low-dose quetiapine treatment may reduce aggression levels and improve sleep quality in adolescents with ASD.