OBJECTIVES: To discuss issues in studying the effectiveness of health services for children, suggest areas in which more research is needed, and recommend strategies for future research. PRINCIPAL FINDINGS: Issues that should be considered include the choice of perspective, which will help determine the interventions studied and the measures of effectiveness and cost-effectiveness chosen. Unique challenges in this area include the fact that serious measurable morbidity is relatively uncommon in children, that causal relationships between services and outcomes may be difficult to establish, and that standard measures of cost-effectiveness may fail to accurately measure important benefits, such as reduced parental anxiety. More research is needed on high-risk and health-promoting behaviors, on critical parent behaviors, on classifying children by vulnerability status, on modes of delivery of preventive care, and on violence prevention. RECOMMENDATIONS: Group-randomized designs and observational research designs that take advantage of natural variations in practice may be increasingly useful in effectiveness studies. Parent- and patient-reported measures of health status and quality of life should be made briefer and more practical for routine use, and better measures of cost-effectiveness are needed. Future research efforts can best be supported by the concerted efforts of various constituencies, including health plans, providers, patients, researchers, and the government.