We studied the effects of two chest physiotherapy regimens on whole lung and regional tracheobronchial clearance (TBC) in ten patients with cystic fibrosis. The regimens were given on two separate days and consisted of 20 min of (1) postural drainage and the forced expiration technique (PD + FET), and (2) positive expiratory pressure (PEP-mask) and FET (PEP + FET). A third day served as control. The study days were randomized. Each day, the clearance of lung radioactivity was measured for 3 h by gamma camera. The number of spontaneous coughs was recorded and the sputum expectorated was sampled. We found that both PD + FET and PEP + FET improved whole lung TBC at 30 minutes and 1 h four or fivefold (p less than 0.01) compared with control, whereas at 2 h and 3 h only the improvement following PEP + FET (approximately 1.4 times) was significant (p less than 0.05). There was no significant difference in whole lung or regional TBC between the PD + FET and PEP + FET treatments. The correlations between TBC and the radioactivity content in sputum expectorated (rs2 = 0.76) and between TBC and numbers of coughs (rs2 = 0.65) were better than between TBC and the weight of sputum expectorated (Rs = 0.39). We conclude that PD or PEP when combined with FET have similar effects on short-term whole lung and regional TBC in patients with cystic fibrosis. Evaluation of TBC during chest physiotherapy when only based on the weight of sputum expectorated seems inadequate.