The single breath test for the measurement of respiratory system resistance and compliance in newborns consists of an end inspiratory occlusion which is subsequently released, allowing expiration to proceed through a pneumotachograph (PNT). The measured flow is then integrated to give volume. The simplicity of the test is one of the major reasons for its popularity. However, some investigators have cautioned against the use of an occlusion distal to the PNT because pressurization of the PNT may introduce artifacts in the flow measurement. Despite this caution, many commercial systems use a pressurized PNT. This study investigated the errors that would result from pressurization of the PNT by providing a step function of flow to two infant PNTs, a Fleisch #0 and a Hans Rudolph 4500, in the unpressurized and pressurized state. In each case there was an initial rapid rise of the flow signal, followed by some overshoot and oscillations that rapidly died out. The overshoot and oscillations for the Hans Rudolph PNT were greater when pressurized whereas pressurization had little effect on the Fleisch PNT. Unpressurized, the two were similar. In either case, the artifact introduced by pressurization of the PNT died out so quickly that it would have little effect on any measurement in an infant.