BACKGROUND: Undernourished children have poor levels of development that benefit from stimulation. Zinc deficiency is prevalent in undernourished children and may contribute to their poor development. OBJECTIVE: We assessed the effects of zinc supplementation and psychosocial stimulation given together or separately on the psychomotor development of undernourished children. DESIGN: This was a randomized controlled trial with 4 groups: stimulation alone, zinc supplementation alone, both interventions, and control (routine care only). Subjects were 114 children aged 9-30 mo and below -1.5 z scores of the National Center for Health Statistics weight-for-age references who were recruited from 18 health clinics. Clinics were randomly assigned to receive stimulation or not; individual children were randomly assigned to receive zinc or placebo. The stimulation program comprised weekly home visits during which play was demonstrated and maternal-child interactions were encouraged. The supplementation was 10 mg Zn as sulfate daily or placebo. Development (assessed by use of the Griffiths Mental Development Scales), length, and weight were measured at baseline and 6 mo later. Weekly morbidity histories were taken. RESULTS: Significant interactions were found between zinc supplementation and stimulation. Zinc benefited the developmental quotient only in children who received stimulation, and benefits from zinc to hand and eye coordination were greater in stimulated children. Zinc supplementation alone improved hand and eye coordination, and stimulation alone benefited the developmental quotient, hearing and speech, and performance. Zinc supplementation also reduced diarrheal morbidity but did not significantly improve growth. CONCLUSION: Zinc supplementation benefits development in undernourished children, and the benefits are enhanced if stimulation is also provided.