Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an inherited deficiency that may be the cause of neonatal hyperbilirubinemia, as has been found in several countries and among widely different ethnic groups, especially in Mediterranean region. Our aim was to study the prevalence of G6PD deficiency in relation to neonatal jaundice.From March 1998 to April 2001 we studied 705 clinically icteric neonates who were admitted to Al-Zahra and Beheshti hospitals, two teaching hospitals in Isfahan, Iran. Laboratory investigations included determination of direct and indirect serum bilirubin concentrations, blood group typing, direct coomb's test, hemoglobin, blood smear, reticulocyte count and G6PD level.In only 53 (7.5%) of cases G6PD deficiency was diagnosed. In all G6PD deficient neonates no evidence of other factors known to cause hyperbilirubinemia were detected. The sex distribution was 13 (24.5%) females and 40 (75.5%) males in the G6PD deficient group. The mean bilirubin level in G6PD deficient and G6PD normal groups were 22.26 +/- 8.36 and 18.14 +/- 3.85 mg/dl, respectively (p=0.001). Phototherapy was required in G6PD deficient and other icteric neonates with duration of 3.76 +/- 1.93 and 3.13 +/- 2.14 days, respectively (p=0.045). Twenty-seven of the 53 (50.9%) G6PD deficient infants required exchange transfusion. None of them developed kernicterus.Since the prevalence of severe hyperbilirubinemia among our neonates was relatively high and about half of them required exchange transfusion, early detection of this enzymopathy regardless of sex and close surveillance of the affected newborns may be important in reducing the risk of severe hyperbilirubinemia and exchange transfusion.