OBJECTIVE: To determine the frequency and interhospital variation of bilirubin testing and identified hyperbilirubinemia in a large health maintenance organization. DESIGN: Retrospective cohort study. SETTING: Eleven Northern California Kaiser Permanente hospitals. SUBJECTS: A total of 51,387 infants born in 1995-1996 at >/= 36 weeks’ gestation and >/= 2000 g. MAIN OUTCOME MEASURE: Bilirubin tests and maximum bilirubin levels recorded in the first month after birth. RESULTS: The proportion of infants receiving >/= 1 bilirubin test varied across hospitals from 17% to 52%. The frequency of bilirubin levels >/= 20 mg/dL (342 micromol/L) varied from .9% to 3.4% (mean: 2.0%), but was not associated with the frequency of bilirubin testing (R(2) = .02). Maximum bilirubin levels >/= 25 mg/dL (428 micromol/L) were identified in.15% of infants and levels >/= 30 mg/dL (513 micromol/L) in .01%. CONCLUSIONS: Significant interhospital differences exist in bilirubin testing and frequency of identified hyperbilirubinemia. Bilirubin levels >/=20 mg/dL were commonly identified, but levels >/= 25 mg/dL were not.