We measured 8-isoprostane, a biomarker of oxidative stress, and prostaglandin (PG) E(2) in exhaled breath condensate in 36 stable and 14 unstable cystic fibrosis (CF) patients, and in 15 healthy age-matched controls. We studied the relationships of these eicosanoids with clinical, radiological, and systemic inflammatory parameters. Compared with controls [15.5 (11.5-17.0) pg/ml] exhaled 8-isoprostane was increased in stable CF patients [30.5 (25.3-36.0) pg/ml, P<0.001]. Unstable CF patients had higher exhaled 8-isoprostane levels [47.5 (44.0-50.0) pg/ml, P<0.001] than stable CF patients. Unlike PGE(2), exhaled 8-isoprostane was negatively correlated with FEV(1) (r=-0.67; P<0.0001; r=-0.63; P<0.02) and Shwachman score (r=-0.43, P=0.012; r=-0.58, P=0.031) and positively correlated with Chrispin-Norman score (r=0.51, P<0.002; r=0.56, P=0.039) in stable and unstable CF patients, respectively. No correlation was observed with C-reactive protein. Compared with controls [41.0 (29.0-50.0) pg/ml], exhaled PGE(2) was also elevated in stable [72.0 (64.3-81.8) pg/ml, P<0.001) and, to a greater extent, in unstable CF patients [83.0 (74.3-91.3) pg/ml, P<0.001). In patients with CF, exhaled 8-isoprostane and PGE(2) could be a useful marker of disease severity.