Infant lung function can be assessed with the tidal volume "squeeze" technique or, over an extended volume range, with the newer raised volume forced expiration technique (RVFET). We assessed methacholine responsiveness in 11 infants, measuring both maximal expiratory flow at functional residual capacity (V.max,FRC)with the tidal volume technique, and forced expiratory volume/time (FEV(t)) with RVFET. We used a standard methodology for the former. FEV(t) was measured by inflating the infant's lungs to 20 cm H2O and forcing expiration using a jacket setup to transmit a pressure of 20 cm H2O to the airway. Lung function was measured at baseline and after methacholine inhalations, increasing from 0.1 g/L to 10 g/L in half log dosage increments (DI). The provocative concentrations (PC) of methacholine leading to a 40% fall in V.max,FRC and a 15 or 20% fall in FEV(t) were calculated. The mean provocative concentration of methacholine required to produce a 40% fall in V.max,FRC was less than that required to produce a 20% fall in FEV0.5 by 0.39 DI (95% CI, -0.60 to 1.38) and less than that required to produce a 20% fall in FEV0.75 by 0.42 DI (95%, CI, -0.54 to 1.39). Similarly, the provocative concentration of methacholine required to produce a 40% fall in V.max,FRC was less than that required to produce a 15% fall in FEV0.5 by 0.14 DI (95% CI, -0.99 to 1.28) or a 15% fall in FEV0.75 by 0.13 DI (95% CI, -0.80 to 1.08), but the differences were small and not significant. Despite these differences the agreement between the two methods was good, and bronchoconstriction was not attenuated by the forced inspiration delivered by the raised volume maneuver. We conclude that the raised volume forced expiration technique is able to detect methacholine-induced bronchoconstriction.