OBJECTIVE: To investigate the significance of jaundice noted in the first 24 hours after birth in a community setting. DESIGN: Supplementary analyses of a nested case-control study. SETTING: Northern California Kaiser Permanente Medical Care Program. PATIENTS: Six hundred thirty-one randomly selected newborns (controls) and 140 cases with total serum bilirubin levels of 25 mg/dL (428 micro mol/L) or higher from a cohort of 105 384 newborns of at least 2000 g birth weight and at least 36 weeks’ gestational age, born between January 1, 1995, and December 31, 1998. MAIN OUTCOME MEASURES: Notations of jaundice in the medical record, timing and results of bilirubin testing, use of phototherapy, and development of bilirubin levels of 25 mg/dL or higher. RESULTS: Among the controls, the cumulative probability of a notation of jaundice (corrected for early hospital discharge using survival analysis) was 2.8% within 18 hours and 6.7% within 24 hours. In these newborns, cumulative proportions that had bilirubin levels measured were 38% within 12 hours and 43% within 24 hours of when jaundice was first noted. About 40% of bilirubin levels measured within 24 hours were above the estimated 95th percentile for age. Compared with newborns not noted to be jaundiced on the first day, newborns noted to be jaundiced within 24 hours were more likely to receive phototherapy (18.9% vs 1.7%; relative risk, 10.1; 95% confidence interval, 4.2-24.4) and to develop a bilirubin level of 25 mg/dL or higher (odds ratio, 2.9; 95% confidence interval, 1.6-5.2), but the absolute risk increase for total serum bilirubin levels of 25 mg/dL or higher was 0.2%. CONCLUSION: Jaundice noted in the medical record in the first 24 hours after birth was uncommon and often clinically significant in this setting, but other factors also need to be considered in determining its importance.