This study examined the effect of the standing versus the sitting position on spirometric indices in 94 healthy non-obese adult subjects (41 men and 53 women) with the order of testing randomised. On average all the spirometric indices examined, except the peak expiratory flow rate, were higher in the standing compared to the sitting position although the change was only significant at the 5% level for FEV1 in women. The fall in FEV1 with the change in position was statistically related to the ponderal index but not to age, height or the initial lung function level. A uniform posture for spirometry is recommended in epidemiological studies examining longitudinal trends in lung function, as well as in cross-sectional aetiological studies examining, for instance, the effects of environmental and/or occupational exposures. A uniform posture is also recommended for clinical studies which involve repeated measurements over time, for instance to measure the effect of treatment or the natural history of airways disease.