The frequency content of airway pressure and gas flow in mechanically ventilated infants (MVIS) has not been adequately investigated. Pressure-cycled infant ventilators generate pressure pulses with short rise-times. Gas flow is approximately equal to the derivative of pressure when lung compliance is low, and hence contains high-frequency components. We defined bandwidth as that frequency fm below which 99.9% of the energy of the signal resided. Simulation of the measurement process using measurement systems with frequency response similar to sixth-order Bessel filters and a lung model comprising series resistance, inertance and compliance showed that measurement systems with frequency response flat +/- 10% to fm yield time domain errors less than 3% of the peak value. We digitized pressure and flow signals from 10-20 ventilator (Healthdyne 105) breaths in 33 stable MVIS. The transducers' (Gould P50, Hans Rudolph 8300 screen pneumotach) frequency responses had been measured between 1 Hz and 100 Hz and phase matched at 10 Hz. We calculated total respiratory resistance R and elastance E using multiple linear regression, and ensemble-average power spectral density using the FFT with a rectangular time window and padding to 2048 points. Power spectra were compensated for non-unity transducer and anti-alias filter responses up to 60 Hz. Measured data sequences that were not self-windowing due to spontaneous breathing efforts, that yielded regression R2 < 0.95 or that contained flow oscillations due to secretions in the airway were discarded. Satisfactory results were obtained from more than eight breaths in 18 infants. Mean bandwidths (+/- SD) of pressure and flow waveforms were 4.7 +/- 0.7, range 3.5-5.9 and 19.6 +/- 6.5, range 10.8-32.1 Hz, respectively. Flow bandwidths B correlated with the respiratory time constant tau (B = -77.2 tau + 26.8, R2 = 0.55, P < 0.0002), and with elastance E (B = 61.4E + 10.1, R2 = 0.74, P < 0.0001). We conclude that the bandwidth of the flow waveform increases with decreasing compliance and mechanical time constant. The frequency response of pressure and flow measurement systems should be flat +/- 10% at least up to 6 and 32 Hz respectively to obtain data with dynamic errors less than 3% in infants with low-compliance lung disease.