Much of the research on determinants of health service utilization has focused on economic and cognitive variables which influence preventative health behavior. Our ethnographic study of maternal perceptions of the barriers and incentives to immunization use in Haiti underscores the importance of 'hidden' social and psychological costs of utilization, such as embarrassment, fear, child care difficulties, and competing demands on maternal time. Findings from focus group interviews with mothers, individual interviews with health care providers, and observation at health posts identified five categories of maternal factors (competing priorities, low motivation, socioeconomic constraints, fears about health or social consequences, knowledge and folk beliefs) and five categories of system factors (accessibility, acceptability, availability, accommodation, affordability) which can deter immunization completion. The discussion focuses on how these factors influence maternal decision-making regarding use of preventive child health services. More attention is needed on the psychosocial costs of health behavior in developing country settings.