The regional distribution of ventilation (Vr), perfusion (Qr) (expressed in absolute values) and the regional distribution of ventilated lung volume, of ventilation and of perfusion (expressed as %) were studied using Xenon 133 in 13 subjects with a unilateral pleural effusion of proven or presumed tuberculous origin. The examinations were performed in a sitting position. The initial examination was carried out after as complete an evacuation as possible of the fluid effusion. Eight subjects underwent a follow-up examination after two months treatment with tuberculostatic agents combined with daily respiratory physiotherapy. The initial results emphasized the following points: - in absolute values, a deficit in ventilation on the involved side affecting the 3 regions explored without any similar change in perfusion ; a hyperventilation on the controlateral upper and middle regions ; - in relative values, a slight deficit in ventilated volume and perfusion localised at the base, whilst the deficit in ventilation was more marked and more extensive. After treatment the contribution to ventilation of the involved side increased on average by 6%. However this improvement only concerned the lower territory. The mechanisms of impairment of regional function and its improvement under the influence of treatment are discussed.