To determine whether lack of medical insurance was associated with adverse health outcomes, this study examined hospital data on newborns in California's San Francisco Bay Area. The study also sought to determine which ethnic groups were most at risk. Computerized data on all civilian acute-care hospitalizations in the study area were obtained for the last half of 1982 and all of 1984 and 1986. The sample included records on 28,345 newborn hospitalizations in 1982; 57,492 in 1984; and 60,631 in 1986. Adverse outcomes were classified as: (1) prolonged length of stay (6 days or more); (2) transfer to another acute-care hospital or long-term care facility; or (3) death. Newborns with adverse outcomes were compared with well newborns (those discharged after a stay of less than 6 days). Analyses of data revealed that in each year studied the risk of an adverse hospital stay appeared higher for uninsured infants than for privately insured newborns. Uninsured black newborns were 2.24 times more likely as privately insured black newborns to experience adverse outcomes. For uninsured Latino newborns the risk was 1.56 times greater. Both the percentage of uninsured infants and the magnitude of risks associated with a lack of insurance increased over time. Findings indicate a need for responses to the lack of health insurance in California and the United States. (MM)