In this study of hospital discharge data on births to residents of an eight-county region of California, we found an increasing lack of health insurance that was associated with an elevated and increasing risk of adverse outcomes in newborns. Between 1982 and 1986, the percentage of newborns without health insurance increased overall by 45 percent (from 5.5 to 8.0 percent; P less than 0.001); the increases were larger among Asians (by 54 percent [from 7.8 to 12.0 percent]; P less than 0.001) and Latinos (by 140 percent [from 8.2 to 19.7 percent]; P less than 0.001). By 1986, the odds ratio for an adverse hospital outcome (defined as a prolonged hospital stay, transfer of the newborn to another institution, or death) was 1.31 (95 percent confidence interval, 1.17 to 1.46) in uninsured as compared with privately insured newborns, with control for race or ethnic group. There was a significant increase in risk over time (P less than 0.03); the comparable odds ratios in 1982 and 1984 were 1.11 (95 percent confidence interval, 0.93 to 1.33) and 1.19 (1.05 to 1.35), respectively. In 1986 the risks were especially elevated for uninsured as compared with privately insured blacks and Latinos (odds ratios, 2.24 [95 percent confidence interval, 1.60 to 3.13] and 1.56 [1.26 to 1.94], respectively); the increases in risk over time were the most marked in these groups. We believe that the elevated and increasing risks for uninsured newborns are explained at least in part by inadequate and diminishing access to care and that this burden is borne disproportionately by blacks and Latinos.