Aims: To study breath pentane exhalation in ventilated preterm infants as a marker of oxygen derived free radical induced lipid peroxidation. Is there a relationship between breath pentane and outcome?Subjects: Ventilated, preterm infants (32 weeks and under).Measurements: Timed breath samples collected into Tedlar bags following washout with hydrocarbon' free air. 96 samples were collected during the first 6 days. Pentane was measured using gas chromatography with thermal desorption and cryofocussing. Results: 34 subjects,mean gestation 26.6 weeks, bwt 943g. 10 babies died, 13 developed CLD, 18 had grade 1-4 intraventricular haemorrhage (IVH). Pentane levels were lowest on day 1(median 6.1 pmol/kg/min) and peaked on day 3 (median 10.4 pmol/kg/min p=0.0085). Maximum pentane was inversely correlated with increasing gestational age (r=-0.68 p<0.001). Infants with IVH had higher maximum breath pentane than those who did not (p= 0.0013) as did infants with retinopathy (p=0.037). Conclusion: The most immature infants had the highest levels of breath pentane. Levels were significantly higher in those with IVH and retinopathy.