To compare incidence, severity, and treatment of jaundice in ABO-compatible and -incompatible infants.Retrospective chart review.A 340-bed acute-care public and teaching hospital.All infants with blood groups A or B delivered in 1990 by Rh-positive mothers with blood group O. On direct antiglobulin testing 65 infants had positive and 78 had negative results. The control group comprised 78 infants with blood group O born to mothers of blood group O.None.Infants who were ABO-incompatible and showed positive results to the direct antiglobulin test had the highest incidence of jaundice and underwent more tests and phototherapy. Infants who showed negative results to the direct antiglobulin test had jaundice incidence rates between those seen for control infants and infants who tested positive. Mean peak bilirubin levels did not differ significantly among the groups.Incompatibility of ABO blood group is associated with an increased incidence of jaundice and higher phototherapy rates. Clinicians should continue to be concerned about possible ABO incompatibility in infants with jaundice born to mothers with blood group O.