Exhaled nitric oxide (eNO) is elevated in patients with asthma in contrast to healthy subjects, although the variability is high. In this study, we tried to reduce the variability of eNO in healthy subjects. We measured eNO using ERS guidelines with a fixed exhalation flow of 250 ml/s in 117 (72 women, 45 men) non-smoking healthy subjects and correlated this to antropometric data and standard lung function measurements. Using a model previously defined by Hyde et al., we selected parameters that were likely to have a high correlation with eNO. ENO was log-normally distributed. The normal values for eNO are significantly (P < 0.001) different for men and women: in women mean ln eNO levels (SD) were 1.49 (0.34), in men 1.74 (0.41) (back-transformed value 4.43 resp. 5.73 ppb). Using multiple regression analysis, only In D(m,CO), InTLC and In sG(aw) showed a significant positive correlation with In eNO in men, although only 20% of the variability of eNO could be explained. In women no correlation was observed and only 5% ofthe variability was explained. The high variability of eNO could only partly be explained in men, which makes the use of reference equations not very helpful.