Autistic Disorder is a condition that develops early in childhood and persists throughout life. Seventy-five percent of children and adolescents with autistic disorder have irritability symptoms such as aggression towards others, deliberate self-injurious behavior, temper tantrums, and quickly changing moods. These symptoms affect their daily functioning such as school performance, interactions with family members and compliance to treatment. Risperidone is an atypical antipsychotic agent that has been recently approved for the treatment of irritability associated with Autistic Disorder in children and adolescents aged 5 to 16 years. The approved dose range is 0.5-3 mg per day. The aim of this study is to evaluate the effectiveness (change in level of irritability and related behaviors) of a lower dose (0.125 mg or 0.175 mg risperidone per day depending on body weight). The study will include three treatment groups. A placebo group, a low dose risperidone group and a higher dose risperidone group (1.25 mg or 1.75mg per day depending on body weight). This phase of the study will be 6 weeks. During the study, neither investigators nor the patients will be told which treatment the patient received. This is called "double blind". The placebo treatment is not expected to be effective. The higher dose group is expected to be effective. At the end of the study, data from the lower dose group will be compared to the placebo group to see if it is effective. Another aim of this study is to evaluate the safety and tolerability of risperidone. At the end of the 6-week double-blind period, patients may enter a 6-month open-label period during which all patients will receive risperidone. During this phase of the study, the doses can be adjusted to a maximum of 1.25 mg or 1.75mg per day depending on body weight. Both investigator and the patient will know what dose the patient is taking. About 93 patients will be randomized. The study will be conducted by investigators from about 15 clinics. Assessments of effectiveness include the Aberrant Behavior Checklist (ABC) subscales including the irritability subscale (ABC-I), the Clinical Global Impression of Change (CGI C); the Clinical Global Impression of Severity (CGI-S); the response rate, and the Compulsions Subscale of the Children's Yale-Brown Obsessive Compulsive Scale (CY BOCS). Safety evaluations include monitoring of adverse events, physical examinations, clinical laboratory tests, nighttime sleep quality and daytime drowsiness, and extrapyramidal symptoms (EPS) as assessed using the Abnormal Involuntary Movement Scale (AIMS), the Barnes Akathisia Rating Scale (BARS) and the Simpson-Angus Scale (SAS). Venous blood samples will be collected for the determination of plasma concentrations of risperidone and 9-hydroxyrisperidone. The study hypotheses are that the higher dose level of risperidone is significantly superior to placebo as measured by change from baseline on the ABC-I Subscale score at end point (Week 6 or early withdrawal) and that the lower dose level of risperidone is significantly superior to placebo as measured by change from baseline on the ABC-I Subscale score at end point (Week 6 or early withdrawal). Double-blind phase: Risperidone oral solutions taken once daily. Depending on body weight patients take 1.25 mL or 1.75 mL of either a 0.1 mg/mL or a 1.0 mg/mL risperidone solution or matching placebo, for 6 weeks. Open-label phase: Medication can be taken once or twice a day. Starting from 0.125mg or 0.175mg per day, drug levels are titrated over 2 weeks to a maximum dose level of 1.25 mg risperidone/day or 1.75 mg /day depending on body weight, for 26 weeks.