Purpose: To assess the variation in thymic size in healthy neonates by sonography and to study the possible correlation to clinical variables. Material and Methods: A study was made of 149 healthy term infants, at less than one week of age. The size of the thymus was assessed by sonography as a volume estimate, the thymic index. This index was compared to sex, weight, length, gestational age, and level of perinatal asphyxia of the infant. T-cell subsets (percentages of CD4 and CD8 receptor-positive T-lymphocytes in peripheral blood) were determined in 83 of the infants and compared to the sonographic thymic index. Results: The thymic index varied between 4 and 29, and was positively correlated to the weight of the infant (p=0.0003). There was no correlation to sex, length, gestational age or slight perinatal asphyxia. We found no correlation between the size of the thymus and the CD4 percentage, the CD8 percentage, or the CD4/CD8 ratio. Conclusion: The size of the thymus in healthy neonates as measured by sonography is significantly correlated to the weight of the infant. For a given weight of an infant it is possible to predict the normal range of thymic size.