Failure of patient triggered ventilation in very premature infants may reflect the use of inappropriate triggering systems. We have therefore compared the performance of an airway and a body surface trigger in 12 infants of median gestational age 26 weeks (range 24-27). Airway flow and oesophageal and ventilator pressure changes were recorded during two periods of patient triggered ventilation. From the traces, the degree of asynchrony (inflation extending beyond inspiration), triggering rate, sensitivity (proportion of the infant's breaths detected) and trigger delay (response time) were calculated. Although with both triggering systems there was a high rate of asynchrony, the triggering rate (p < 0.05), sensitivity (p < 0.05) and trigger delay (p < 0.01) were all better with the body surface trigger. These results suggest that the body surface trigger is the more appropriate system for very immature infants.