A simple, practical method is described for estimating unilateral lung function. Overall lung function is multiplied by right-left fractional perfusion estimated from a quantitative technetium ferric hydroxide macroaggregate lung scan. The calculated function attributed to the lung to be resected is then subtracted from the overall pulmonary function. The data from 13 patients indicate that the following postpneumonectomy functions can be predicted: forced vital capacity, forced expiratory one second volume, maximum voluntary ventilation, functional residual capacity, total lung capacity and diffusing capacity. The precedures described are available in most hospitals and represent a practical alternative to classic bronchospirometry, xenon radiospirometry and unilateral pulmonary artery balloon occlusion in the estimation of split lung function.