Low immunization rates among US preschool children suggest a need for improved immunization practices. Immunization and encounter records were reviewed to ascertain immunization rates and missed opportunities for vaccine administration among 515 preschool children who were active patients at a hospital-based primary care center serving lower socioeconomic status families in Rochester, NY. The point prevalence at a mean age of 4.4 years for lack of one or more recommended immunizations was 27%; 7% were missing measles-mumps-rubella, 18% were missing Haemophilus influenzae type b, 8% were missing two or more diphtheria-tetanus-pertussis, and 4% were missing two or more oral poliovirus immunizations. A visit was counted as a missed opportunity if an immunization was due but not given. Over the period from birth through age 36 months, 422 (82%) of children missed at least one immunization opportunity. For these 422 children, there was a mean of 7.2 missed opportunities per child. Although 64% of missed opportunities occurred at an acute illness visit, 36% occurred at well-child, administrative, follow-up, or chronic illness visits. Review of 200 medical records randomly selected from all opportunities at acute illness visits found no contraindication in 63% (50% nonfebrile infectious disease, 13% minor noninfectious problems). Findings for random samples of 100 missed diphtheria-tetanus-pertussis opportunities for children aged 2 to 6 months and 100 missed measles-mumps-rubella opportunities for children 15 to 24 months were similar to findings for the sample of all acute illness visits. Emergency department visits, where immunization records were not readily available, accounted for 18% of missed opportunities.(ABSTRACT TRUNCATED AT 250 WORDS)