Glucose-6-phosphate dehydrogenase (G6PD) deficiency is prevalent in Thailand. This condition can cause acute hemolysis during oxidative stress and also severe hyperbilirubinemia in the newborn in some populations. Our aim was to study the prevalence of G6PD deficiency in relation to neonatal jaundice. We performed quantitative red blood cell (RBC) G6PD assay in the cord blood of 505 male subjects. Observation of jaundice and determination of bilirubin level as well as work up for other causes of jaundice were made in the G6PD deficiency group compared to a G6PD normal group. Questionnaires were also sent for further follow up to both groups. The results of the study were as follows: Sixty-one of 505 male (12.08%) had RBC G6PD deficiency (Group I). The rest (444 cases) had normal G6PD (Group II). In Group I, 49.15% developed neonatal jaundice, of which 28.82% were physiologic and 20.33% were pathologic jaundice. In group II, 23.68% developed jaundice; 16.51% were physiologic and 7.17% were pathologic jaundice, respectively. Onset of jaundice, date of peak bilirubin and peak bilirubin level in Group I and Group II were not statistically different. ABO incompatibility was associated with Group I in 17.24% and with Group II in 9.09%. Hospitalization day in Groups I and II were not statistically different. Other associated diseases were found in both groups, ie infection, congenital malformation, respiratory distress syndrome, but there was no significant difference in terms of jaundice. Phototherapy was required in 18.64% and 10.28% in Group I and II with a duration of 3.91 +/- 1.24 and 3.21 +/- 1.75 days, respectively. One case in Group I who was also premature received one exchange blood transfusion due to severe sepsis but he did not survive. One case in Group II who had polycythemia was successfully treated by partial exchange transfusion with plasma.