To compare two methods of detecting central apnoea, 27 preterm infants (median birth weight 1.14 kg, gestation 29 weeks) were studied on 89 occasions, using combined thoracic impedance and abdominal respiratory inductive plethysmography. During 353 h recording time, 946 apnoeic episodes were noted. In 651 of these, there was agreement between impedance and inductance traces, but apnoea appeared longer in 106 episodes on impedance, and in 189 on inductance. If a single system had been used 27 would have been missed by inductance and 112 by impedance (P less than 0.05). Failure of impedance monitoring was mainly due to technical reasons, especially cardiac artefact, and to movement. The use of a dual system of impedance and inductance offers improved detection of central apnoea, particularly for research purposes.