The effect of personal cigarette smoking on the growth of lung function in children and adolescents has been assessed in a longitudinal study of a group of 669 subjects 5 to 19 yr of age at initial examination. Subjects were seen annually and assessed with standard questionnaires and measurements of forced expiratory volume in one second (FEV1) and forced expiratory flow during the middle half of the forced vital capacity (FEF25-75). Multiple regression analysis revealed that after correction for previous FEV1 or FEF25-75, age, sex, height, change in height, interactions of age and change in height, and sex and height, and mother's smoking, the personal smoking by the children led to a significant decrease in the rate of growth of FEV1 (p less than 0.001) and FEF25-75 (p = 0.033). On the basis of this analysis, it is estimated that, on average, children who begin to smoke at 15 yr of age and continue to smoke would achieve only 92% (95% confidence limit, 87 to 96%) of their expected FEV1 and 90% (95% confidence limit, 81 to 99%), of their expected FEF25-75 at 20 yr of age. These data suggest that relatively small amounts of cigarette use (median total consumption of 7,300 cigarettes in this study) by adolescents can lead to significant effects on the growth of lung function.