Twenty-five patients with diverse pulmonary diseases were studied with chest radiography, spirometry, diffusion capacity measurements, differential spirometry and lung scanning. The differential spirometry was performed at rest and during exercise, and the data were analysed by grouping the patients according to the radiographic findings, spirometry and diffusion capacity.Results of spirometry and DLco correlated poorly with the functional evaluation obtained with differential spirometry. A better relationship was seen between the radiographic evaluation and the resting differential spirometry data. However, radiographic evaluation was not a good index of the functional reserve of a lung. .Individual variations were considerable, and it was not possible to predict the functional reserve of a lung from resting differential spirometry data in individual patients.Lung scans were obtained in nine patients at rest and in seventeen patients during exercise. The correlation between the differential oxygen uptake and the radioactivity of the scan in the lung was found to be excellent. The perfusion of each lung, thus, could be determined at rest as well as during exercise by this method.A simultaneous evaluation of ventilation and perfusion of individual lung and its functional reserve by determination of the same parameters, during exercise, is essential particularly when chest surgery is contemplated. Such information can easily be obtained at present, in our opinion, only by differential spirometry.