• The recurrence of neonatal hyperbilirubinemia (NHB) in full siblings was studied in 3301 live infants born between 1966 and 1986 to 1669 male US Army veterans who were part of a nationwide health study. The study population included 580 sibships with one infant, 679 with two, and 410 with three or more. Hospital of birth medical records were abstracted on these children. Neonatal hyperbilirubinemia was defined as present if the recorded peak bilirubin level was greater than 205 μmol/L in the absence of hemolytic disease of the new-born. The risk of NHB in newborns who have one or more prior sibs with NHB was 3.1 times higher than that of new-borns who have prior sibs without NHB (10.3% vs 3.6%). Simultaneous adjustment for risk factors for NHB, such as feeding patterns, year of birth, maternal obstetric events, and infant health variables, did not explain the excess risk of NHB in sibs. Moreover, the risk of severe NHB (peak bilirubin level, >257 μmol/L) in newborns who have one or more prior sibs with severe NHB was 12.5 times higher than that of newborns who have prior sibs without severe NHB (10.5% vs 0.9%). Separate analyses in sibships where all sibs were breast-fed and in sibships where all sibs were bottle-fed gave similar results. These data clearly suggest the familial nature of NHB. The higher risk of recurrence of NHB in sibs does not seem to be due to known environmental risk factors for NHB.