A GC-MS method for the simultaneous determination of hexanal, heptanal, octanal, nonanal and decanal in exhaled breath was established and validated. The aldehydes were derivatized on PDMS/DVB fibres using O-2,2,4,5,6-(pentafluorobenzyl) hydroxylamine hydrochloride (PFBHA) as the headspace derivatization reagent. The resultant oximes were quantified by GC-MS in selected ion monitoring (SIM) mode. The method provides detection limits of 0.01-0.03 nM for the aldehydes, with a linear response in the concentration range 0.002-20 nM. Within-day precision values for the five aldehydes at 0.02-0.04 nM and 0.2-0.4 nM were in the ranges: 3-9% and 3-8%, respectively; the corresponding between-day precision values were 11-22% and 10-24%. Exhaled breath samples could be stored at -20 degrees C for 48 h.