We studied the acute effects of 4 different chest physical therapy regimens using a randomised cross-over design in 14 patients with cystic fibrosis. Treatment A consisted of postural drainage, percussion and vibration; treatment B of postural drainage and periodic application of a face mask with positive expiratory pressure (PEP); treatment C of PEP in the sitting position; treatment D of the forced expiration technique in the sitting position. In terms of sputum expectorated, treatments B and C were superior to treatment D and especially to treatment A (p less than 0.05). Skin oxygen tension, PSO2 was monitored continuously during and for 35 min after treatment. A substantial and prolonged decay in PSO2 was observed during treatment A, quite different from other patterns seen. During and even following treatment C, an increase in PSO2 was noted. PEP was well accepted by the patients, who preferred treatment C, and we suggest it is incorporated in chest physical therapy regimens if the therapeutic objective is to increase expectoration.