High-frequency oscillation (HFO) has been used clinically to ventilate infants with respiratory distress. However, there are problems in monitoring the effects on the respiratory system and in particular in measuring the volumes delivered; this is important information in terms of safety and mechanisms of action of HFO. We have validated two sizes of respiratory jacket for measuring oscillatory volume changes of 0.25-5 ml at frequencies of 2-25 Hz, the volume delivered from a purpose-built oscillator having first been validated. Different combinations of volume and frequencies were then oscillated into each jacket, while it was being worn by a well preterm baby. Studies were performed with each jacket on five babies with weights between 0.82 and 1.86 kg. The results showed that at any given frequency there was a linear relationship between the pressure oscillations measured from a side port of the jacket and the delivered volume. Both jackets showed the same pattern of frequency response, overreading at less than 10 Hz and underreading at 10-25 Hz. When appropriately calibrated, the respiratory jacket can be used as a non-invasive method of measuring volumes delivered by HFO.