To quantitatively assess the impact of phototherapy (PT) in the prevention of kernicterus we calculated the rate of exchange transfusion (ET) in two large historical cohorts of Greek neonates (birthweight > or = 2.5 kg), one before (period I: 1957-61) and one after (period II: 1980-92) the introduction of PT. Overall, the introduction of PT was associated with a reduction of the rate of ET from 0.43% in period I to 0.05% in period II. The reduction was observed in all etiological categories but was more marked in the ABO-incompatible group. With an estimated rate of kernicterus without treatment of 0.085% (excluding rhesus hemolytic disease of the newborn and prematurity) we estimated that 4.2 and 0.36 infants were treated by ET for each spared kernicterus in periods I and II, respectively. Finally, in period II 185 infants were treated with PT or PT and ET for each spared kernicterus.