To define missed opportunities for immunization (MOI) at Albert Sabin Children's Hospital, in the city of Fortaleza, Brazil, the author studied 119 outpatients aged under 24 months that were attended at the hospital. Thirty-seven children (31%) were missing vaccines. Ninety percent of the children had immunization cards but only 40% had their cards verified by health workers. Fifty-eight percent out of 43 inpatients were missing vaccines without contraindication and 54% of the mothers were missing doses of tetanus toxoid vaccine. Eighty-six percent of 22 health workers had good knowledge about the vaccination schedule, but only 23% did know 3 true contraindications to vaccines. The most important causes of MOI were health workers' neglect to ask for the vaccination card and false contraindications.