The objective was to study the impact of birth weight on the relationship between ambulatory blood pressure and urinary sodium excretion in children and adolescents. The study included 134 healthy children (61 boys), all Caucasians, who were born at term after a normotensive pregnancy. For each subject, a 24-hour ambulatory blood pressure monitoring and a complete urine collection were simultaneously performed according to the protocols designed. Average ambulatory blood pressure (BP) and the urinary excretion rates for sodium, potassium, and creatinine were calculated separately for 24-hour, awake, and sleep periods defined by a mini-diary. The excretion rate of sodium during sleep time was positively correlated with ambulatory systolic BP; such a positive relationship was not found for waking hours. Consequently, the impact of birth weight on the relationship between blood pressure and the urinary sodium excretion rate was analyzed during sleeping hours. Stepwise multiple regression analysis shows that although current weight was the strongest predictor for the sodium excretion rate during sleep (P<.001), there was also an independent significant direct relationship for birth weight (P<.04) after controlling for age, sex, and the average of systolic BP during sleep. Adjusted for current weight, a significant difference in the regression slopes relating urinary sodium excretion rate and systolic BP during sleep exists between children in the lowest (<3.100 kg) and the highest tertiles (>3.500 kg) of birth weight (P<.02). Differences in sodium excretion rates, adjusted for current weight, between the two extreme tertiles of birth weight became significant at the highest systolic BP (P<.04). The children who had the lowest birth weight tended to excrete less sodium during sleep. The results of the present study show a blunted pressure natriuresis curve in children and adolescents with the lowest birth weight. Whether this abnormal renal sodium handling may be present as an initial or as an intermediate mechanism leading to higher BP values must be assessed in additional studies.