In 28 patients with cystic fibrosis aged 5--24 years the values of maximum expiratory flow rates (Vmax) at lower volume levels were assessed, as well as the "specific" conductance of the respiratory pathways (Gaw/TGV), the vital capacity (VC), total lung capacity (TLC), residual volume (RV), functional residual capacity (FRC), RV/TLC and FRC/TLC ratios and the one-second forced expiration of the vital capacity (FEV1) to evaluate obstruction of the respiratory pathways. Most markedly and most frequently abnormal were the values of Vmax, RV and of the RV/TLC ratio. These findings revealed that in almost all patients with CF there was already during the initial examination an obstruction of the peripheral respiratory pathways and hyperinflation of the lungs. During repeated measurement of the above values of lung function in 15 patients with cystic fibrosis during a period of 1--5 years when the patients increased in height by 10 cm on average, the above values did not deteriorate on average. There was, however, an individual variability of the investigated values during this period. The comprehensive treatment provided in our country prevented in patients with CF a deterioration of obstruction of the respiratory pathways during the period of investigation.