BACKGROUND: Missed opportunities to vaccinate occur commonly, contributing to underimmunization of young children. They are related to provider knowledge of the immunization schedule and contraindications to vaccination. METHOD: We surveyed private physicians (n=50) and public health physicians and nurses (n=47). The questionnaire presented two sets of clinical scenarios in which they had to assess what immunizations were due and assess whether there were any contraindications to vaccination. RESULTS: The mean percent correct responses on the immunization schedule questions was 64% for the private physicians, 71% for the public physicians and 78% for the public nurses. The mean percent correct responses on the contraindications to vaccinate questions was 73% for public physicians, 58% for private physicians, and 55% for public health nurses. CONCLUSION: Providers in the public and private sectors have important deficits in knowledge of the immunization schedule and the appropriate contraindications to vaccinate which might lead to missed opportunities to vaccinate and low immunization coverage. (Authors)