The goal of psychopharmacologic research in autism is to provide guidance to clinicians and families on the risks and benefits of specific interventions. Careful subject selection and subject characterization in clinical trials are necessary for replication, to inform clinicians about the sample, and to elucidate the type of patients who might benefit from the treatment. At minimum, subject characterization includes demographic information, diagnosis (autism, Asperger's syndrome, or pervasive developmental disorder-not otherwise specified), intellectual functioning, adaptive functioning, symptom severity, general behavioral profile, health status, pertinent clinical laboratory measures, height, weight, current treatments, and educational placements. Subject selection, sample size, and choice of the primary outcome measure are closely interrelated and linked to the study hypothesis. The magnitude of expected improvement on the primary outcome measure, which can be expressed by effect size, has direct implications for sample size. Large sample sizes are required to detect small effect sizes. To facilitate interpretation of study results, research reports should provide descriptive characteristics of the sample as well as the mean change and standard deviation on the primary outcome measure data to permit calculation of the effect size.