We investigated the effect of a unilateral thoracic expansion exercise (TEE), a breathing manoeuvre used by physiotherapists, on regional lung ventilation. Nine trained physiotherapists aged 22-37 years completed the study. Technegas lung ventilation scans were used to determine the effect of a right unilateral TEE performed when sitting. This was compared with a maximal deep breath. Total radioactivity in each lung was determined. Each lung was sectioned into three equal zones (upper, middle and lower) and the ratio of radioactivity for each of the corresponding lung zones calculated. Ventilation was preferentially distributed to the right lung in all participants during both breathing manoeuvres. The mean(± s.E.M.) radioactivity ratios (right/left lung) were greater during a unilateral TEE (1.17 ± 0.02) than during a deep breath (1.07 ± 0.01). Seven participants achieved significantly greater ventilation to the right middle (1.15 ± 0.03, P = 0.02) and lower zones (1.34 ± 0.03, P = 0.02) during a unilateral TEE than to the corresponding zones on the left; this was evident soon after the initiation of the breath. The findings of this study show that relative regional ventilation to the ipsilateral lung can be increased during a unilateral TEE in trained individuals.