Abstract A questionnaire sent to 40 intensive care units showed that ventilator circuits were changed every 24 h in 62% and every 48 h in 20% of the units. A similar survey in 40 special-care baby units showed that only 9·7% changed circuits daily and the majority were changed weekly (38·7%) or between patients (38·7%). A prospective study of bacterial contamination of circuits in adult patients showed that when water humidification was used, 28 72 samples from inspiratory and 31 72 from expiratory tubing were contaminated with Gram-negative bacilli. None of 48 sets of tubing was contaminated when a heat-moisture exchanger was used and only one expiratory tube was contaminated in tubing from 45 neonates using a water humidifier. Gram-negative bacilli were isolated from the mouths of 21 36 adults and 6 12 neonates, and also from 6 17 hands of staff after changing circuits. It is suggested that circuits with water humidification may be changed every 48 h in adult patients, and between patients or weekly in neonatal units or between patients if a heat-moisture exchanger is used. Handwashing after removal of a circuit is of major importance in the prevention of spread of infection.