The noseclip is conventionally used in lung function testing to prevent leakage via the nasal compartments. However, some subjects exhibit a velum-opening reflex which may affect results.We performed forced oscillation measurements at frequencies (8–256 Hz) that include the first antiresonance, comparing the noseclip with a cotton wool nose plug to eliminate upper airway contribution. Three sets of measurements were made in 18 adults: with and without noseclip, and with cotton wool. Velum opening during noseclip measurements was monitored using a nasal pressure transducer.A significantly greater proportion of subjects produced a characteristic distortion to the first antiresonance with the noseclip than with either no noseclip or with cotton wool. Distortion of the spectrum coincided with the transmission of oscillations into the nasal cavity.Thus, the noseclip cannot be used in high-frequency forced oscillation measurements because of the velum reflex. The cotton wool plug offers a simple alternative. This effect has unknown impact in other lung function tests.