Summary Of 250 cases of unilateral S.P.P.E. in adults 55.6 per cent occurred on the right side and 44.4 per cent on the left. A comparison is made between the effects of treatment of 140 cases treated conservatively and 50 cases treated by repeated aspiration, intrapleural and intramuscular streptomycin with oral I.N.A.H. combined with posture and physiotherapy. The significance is discussed of the identification, on routine full-plate tomography after three months' treatment, of the cryptogenic parenchymal pulmonary lesion (12.79 per cent of 86 consecutive cases). The Connaught Hospital method of treatment is an active form of treatment combined with antibacterial drugs to replace the method of masterful inactivity which rightly has held the field so long but which should now be abandoned.