Undernutrition in early childhood is associated with poor mental development and affects 45% of children in Bangladesh. Although limited evidence shows that psychosocial stimulation can reduce the deficits, no such interventions have been reported from Bangladesh. The Bangladesh Integrated Nutrition Program (BINP) has provided nutrition supplementation to undernourished children through community nutrition centers (CNCs). We added psychosocial stimulation to the treatment of undernourished children in a randomized controlled trial to assess the effects on children's development and growth and mothers' knowledge. Twenty CNCs were randomly assigned to intervention or control groups with 107 children in each group. We also studied 107 nonintervened better-nourished children from the same villages. Pre- and postintervention measurements included children's height, weight, development assessed on Bayley Scales, behavior ratings during the test, and a questionnaire on mothers' knowledge of childrearing. The intervention comprised home visits and group meetings with mothers and children for 12 mo. Intervention benefited children's mental development (4.6 +/- 2.0, P = 0.02), vocalization (0.48 +/- 0.23, P = 0.04), cooperation (0.45 +/- 0.16, P = 0.005), response-to-examiner (0.50 +/- 0.15, P = 0.001), emotional tone (0.33 +/- 0.15, P = 0.03), and mothers' knowledge (3.5 +/- 0.49, P < 0.001). At the end, undernourished controls had poorer mental (-4.6 +/- 2.0, P = 0.02) and motor (-6.6 +/- 2.2, P = 0.003) development, were more inhibited (-0.35 +/- 0.16, P = 0.03), fussier (-0.57 +/- 0.16, P < 0.001), less cooperative (-0.48 +/- 0.17, P = 0.005), and less vocal (-0.76 +/- 0.23, P = 0.001) than better-nourished children. Intervened children scored lower only in motor development (-4.4 +/- 2.3, P = 0.049). Neither group of undernourished children improved in nutritional status, indicating that treatment had no effect. In conclusion, adding child development activities to the BINP improved children's development and behavior and their mothers' knowledge; however, the lack of improvement in growth needs to be examined further.