OBJECTIVE: Research into cognitive development and behavior in children undergoing heart or heart-lung transplantation has focused mainly on older children, with investigation of children undergoing transplantation as infants or toddlers being largely retrospective. This study was conducted, therefore, to obtain pretransplant baseline measures of development and behavior for preschool-aged children. METHODS: Children <3.5 years old were assessed before transplantation (n = 35) and compared with a group of children awaiting conventional cardiac surgery, a group undergoing bone marrow transplantation, and a group of healthy children. Development was measured by using the Ruth Griffiths Mental Development Scales, and behavior was measured with the Achenbach Child Behavior Checklist. RESULTS: Within the transplant group, 17 had congenital heart disease (CHD), and 18 had cardiomyopathy (CM). Although the overall mean developmental scores were within the normal range for the transplant, conventional cardiac surgery, and bone marrow transplantation groups, scores were significantly lower than those of the healthy group. Within the transplant group, those with CHD had a significantly lower mean developmental quotient than those with CM. Furthermore, the CHD patients obtained significantly lower scores than those with CM on areas of development covering locomotor abilities, speech and hearing, eye-hand coordination, and performance. CONCLUSIONS: In common with other groups of ill children, patients awaiting heart or heart-lung transplantation are at risk for developmental delay. Diagnosis is a salient factor in determining outcome in most areas of development. Psychosocial interventions need to be targeted to maximize developmental potential before transplantation.