The use of 13NN and positron imaging provides a powerful noninvasive means of assessing regional pulmonary function. Current techniques for analyzing lung washout curves, however, are subject to error due to intraregional nonuniformities, particularly in the presence of areas with very long time constants or gas trapping. This paper presents a simple "hybrid" method for analyzing 13NN-washout studies that addresses the problems of regions of air trapping and long time constants within a region of interest. This method assumes an exponential washout form for the slow regions, estimates their fractional volume, and subtracts their contribution from the overall washout curve. The modified Stewart-Hamilton method is then applied to the remaining washout curve to calculate its ventilation. The over-all specific ventilation of the region is calculated as the volume-weighted average of the specific ventilations of the two compartments. The performance of the hybrid method is compared with other currently used correction techniques by using a simulated two-compartment lung washout in which there is a variable amount of gas trapping. The analysis techniques are also applied to washout data with intraregional nonuniformities obtained during experimentally created unilateral bronchial obstruction. This new approach has the advantages of automatically correcting for washout truncation, estimating the relative size of the trapped or slow region while eliminating its bias on the regional ventilation, and yet retaining the unrestricted and robust nature of the Stewart-Hamilton method in the analysis of the well-ventilated regions.