The Fowler technique for estimating anatomic dead space (VD -- the volume of the airway conducting system) on the basis of CO2 changes in the expired air of a single breath was adapted to computer analysis. The areas above and below the sigmoid CO2 curve in the region of transition from inspired to alveolar concentration were simultaneously integrated (forward and backward) on a time base until the sum of the two integrated values equalled the total area representing the transition. The point on the CO2 curve was defined as the CO2 midpoint. Expiratory flow was then integrated with time from its beginning to the CO2 midpoint, and the resultant volume represented VD for that breath. The computer program was able to compensate for differences in response time between the gas monitor and the flow measuring device and was able to integrate the areas rapidly. The program provided a rapid calculation of VD, which compared well with values calculated by tedious, manual methods.