'Idiopathic' hyperbilirubinaemia in the first 4 days of life was studied in 431 unselected healthy full-term (gestational age > or = 37 weeks) singleton Sardinian infants with birthweight > or = 2500 g. All infants were free from malformations or any disease requiring treatment other than jaundice, they were ABO and Rh compatible with their mothers and were not G6PD deficient. The serum bilirubin level was > 11.9 mg/dl (204 mumol/l) and > 14.9 mg/dl (256 mumol/l) in 37.1% and 15.3% of the study subjects. The vast majority of the infants (94%) were breast fed; no difference in the incidence of jaundice was found between breast-fed and bottle-fed infants. A logistic regression analysis indicated that high alpha-fetoprotein concentrations in cord blood, history of neonatal jaundice in previous full-term siblings, delayed first meconium passage and weight loss were associated with jaundice, defined as a serum bilirubin level > 11.9 mg/dl. These results suggest that the high rate of neonatal hyperbilirubinaemia in Sardinia is mostly related to constitutional and possibly hereditary factors.