In 10 normal volunteers, total lung capacity determined from a single-breath N2 washout was not significantly different from that determined by body plethysmography. In patients who underwent clinical pulmonary function tests, total lung capacity was substantially underestimated by the single-breath N2 washout, compared with that determined by either body plethysmography or the 7 min open-circuit N2 washout method. The single-breath N2 washout underestimated total lung capacity in patients who had a normal slope of Phase III, and the error was even greater in patients who had steeper Phase III slopes or low maximal mid-expiratory flow values. Total lung capacity determined by a 5 vital capacity breath N2 washout method was comparable to that determined by the 7 min N2 washout method, provided that expired N2 did not exceed 10% during the 5th vital capacity maneuver. By extending the washout beyond 5 vital capacity maneuvers when necessary to achieve a peak expired N2 of less than 10%, a mean total lung capacity within 1% of that determined by the 7 min washout method was obtained. The multiple vital capacity maneuver was well tolerated by patients and saved considerable time.