In a population-based study including 2463 infants, serum bilirubin measurements were added to the neonatal screening programme for phenylketonuria and congenital hypothyreosis. This screening programme detected 11/17 (65%) of infants with serum bilirubin levels >350 micromol 1(-1), of whom 7 (3 per 1000) were readmitted from home (6 treated with phototherapy). A total of 139 infants (5.6%) received phototherapy. Maternal blood type O occurred significantly more often in term infants treated (30/54; 55.6%) compared with preterm infants treated (32/85; 37.6%) and with blood type O occurrence in the total population of mothers (906/2426; 37.3%) (p < 0.05). The blood type constellations mother O/infant A or B showed a sensitivity of 64%, specificity 65%, positive predictive value 12% and a negative predictive value of 96% for the requirement of phototherapy for the whole material. Exchange transfusion was not required in any of the infants. No infant developed bilirubin encephalopathy (kemicterus). Adding bilirubin to a neonatal screening programme detects some cases with unexpectedly high bilirubin levels in need of intervention. Routine ABO blood typing of pregnant women, ABO cord blood typing and Coombs' test in infants of mothers with blood type O cannot be recommended because of low positive predictive value for the requirement of intervention (phototherapy) by these tests.