['To determine whether changes in aerobic fitness and body mass index are related to the age-related rise in blood pressure in healthy preschool children.', 'Longitudinal analyses of 196 free-living children aged 5 years at baseline who were followed over a mean of 19.7 months. Aerobic fitness was assessed using a treadmill. All measures were obtained on multiple occasions at scheduled visits as part of a longitudinal cohort study.', 'An inner-city medical center.', 'Blood pressure was measured using an automated Dinamap device.', 'Mean systolic blood pressure was 95.3 mmHg (SD 8.38) at baseline and increased by 4.46 mmHg per year. Mean diastolic blood pressure was 53.9 mmHg (SD 5.81) at baseline and did not change significantly. Children in the highest quintile of increase in fitness had a significantly smaller increase in systolic blood pressure compared to children in the lowest quintile (2.92 vs 5.10 mmHg/year; P = .03). Children in the lowest quintile of increase in body mass index did not differ significantly in rate of increase in systolic blood pressure compared to children in the highest quintile (3.92 vs 4.96 mmHg/year). In a multiple regression model including baseline systolic blood pressure, fitness, height, body mass index, and other covariates, greater increase in fitness (P = .03) and lesser increase in body mass index (P < .01) were associated with lower rates of increase in systolic blood pressure. In a similar multivariate analysis, an increase in fitness was also associated with a lower rate of increase in diastolic blood pressure (P = .02).'] Is the rate of increase in blood pressure in children 5 years of age related to changes in aerobic fitness and body mass index?