This longitudinal study was performed in a group of 3-12 year old children to document normal lung growth patterns as measured by spirometry. By clinical and laboratory parameters, these children were free of illness at the time of study and had been for the preceding 21 days. Spirometry was performed prospectively over a period of six years in 69 children (27 black females, 23 black males, 10 white females, and 9 white males) all from a day care center. Eight hundred fifteen spirometric tests were made on these children. Six spirometric parameters were measured: forced vital capacity (FVC), forced expiratory volume in one second (FEV1), peak expiratory flow (PEF), forced expiratory flow during the middle half of the FVC (FEF25-75%), and maximum expiratory flows after 50% and 75% of the FVC have been exhaled (Vmax50% and Vmax75%, respectively). There were significant differences between the regression lines (considering slope and intercept) for all six parameters when black females were compared to black males, white females to white males, black females to white females and black males to white males, except for Vmax75% for the comparison of black females to black males and white females to white males. These slopes and intercepts were similar to those reported by others for children of similar age and height. This study demonstrates that spirometry can be performed reliabley at an early age in a day care center population, that there are significant racial and sexual differences in spirometric volume and flow parameters and that the variability of these measurements is proportional to height rather than being a constant.