Disclosed is a method and device for performing forced expiratory maneuver in an infant to assess the infant's pulmonary function. Under this method, the infant's lungs are synchronously inflated to super-atmospheric levels synchronous with the infant's natural tidal inspiration for a plurality of consecutive respiratory cycles. The end-expiratory CO.sub.2 levels in the infant's respiration are measured during the test. When the end-expiratory CO.sub.2 concentration decreases from the baseline by a pre-defined amount, the infant lungs are rapidly inflated to substantially total lung volume and rapidly deflated to produce a maximum forced expiration. The pre-defined amount of change in CO.sub.2 concentration is usually determined by the testing clinician. Typical concentration drop in CO.sub.2 levels, for example, ranges between 4 and 8 mmHg. But the decrease may also be as little as 2 mmHg or as much as 15 mmHg, depending on the testing clinician. The decrease in the end-expiratory CO.sub.2 level of the infant indicates that the infant's respiratory center is sufficiently modified to allow for the measurement.