A motivational spirometer developed for mass screening aids in evaluating successive forced expiratory trials by using a minicomputer to calculate for each trial the deviations from predicted values on the ventilatory parameters, to compare successive trials for reliability of best trial parameters as an index of maximal effort, and to detect machine artifacts and misunderstandings on the part of the subjects as to procedure. The motivational aspect of the device consists of a series of 7 lights cued to fractions of the predicted forced vital capacity (FVC). Refinement of linear prediction equations was made on 12 ventilatory parameters. Regression analysis was made of 3,776 females and 2,400 males from a total screening population of 51,234 asymptomatic nonsmokers with satisfactory computerized spriometry tests. Analysis of an additional 327 representative adults yielded the estimated required time for an FVC of the best trial.