The effect of a number of explanatory variables on the degree of physiological jaundice in mature infants was evaluated by multiple regression analysis. The sampling was designed so that comparisons could be made between siblings. We found an effect of gestational age, gender, infant nutrition, ABO incompatibility and induction of labour. Taking these factors into account we still found a highly significant correlation between the peak bilirubin levels of siblings. Whether this correlation is genetically or environmentally determined is not clear.