The purpose of this work was to develop an automated technique for calculating dynamic lung attenuation changes, through a forced expiratory maneuver, as a measure of split lung function. A total of ten patients post single lung transplantation (SLT) for emphysema were imaged using an Electron Beam CT Scanner; three were studied twice following stent placement. A single-slice flow study, using 100 msec exposures and 3 mm collimation, was performed at the level of the anastomosis during a forced expiration. Images were acquired every 500 msec for the first 3 seconds and every second for the last 4 seconds. An automated, knowledge-based system was developed to segment the chest wall, mediastinum, large airways and lung parenchyma in each image. Knowledge of the expected size, shape, topology and X-ray attenuation of anatomical structures were used to guide image segmentation involving attenuation thresholding, region-growing and morphology. From the segmented left and right parenchyma, the system calculated median attenuation (HU) and cross-sectional areas. These results were plotted against time for both the native and transplanted lungs. In five patients, significant shift of the attenuation/time curve to the right (slower flow) was detected, although the end expiration attenuation was not different. Following stent placement the curve shifted back to the left (faster flow).