The aim of the present study was to assess the reproducibility of changes in forced inspiratory volumes after bronchodilator inhalation. Thirteen patients with chronic obstructive pulmonary disease (COPD) (FEV1, 32-75%pred) and 10 patients with asthma (FEV1, 43-75%pred) inhaled either 200 microg fenoterol or 200 microg oxitropium bromide or placebo, each of them on three occasions, on nine different days in a randomised, cross-over, double-blind fashion. Forced expiratory (FEV1) and inspiratory (FIV1) volumes were measured before and 30 min after inhalation. In patients with COPD, the increase in FEV1 (coefficient of variation) was 221 ml (43%) after fenoterol and 235 ml (33%) after oxitropium; changes in FIV1 were 301 ml (45%) and 360 ml (29%). In patients with asthma, FEV1 improved by 618 ml (26%) and 482 ml (25%), FIV1 by 553 ml (41%) and 475 ml (23%). In less severe COPD or asthma, the reduction in dyspnoea was associated with the improvements in both FIV1 and FEV1, but in severe COPD with the improvement in FIV1 only. The data demonstrate that, at least in terms of relative changes, the reproducibility of bronchodilator responses in terms of FIV1 is similar to that of FEV1 and they underline the assertion of FIV1 being a sensible parameter particularly in severe COPD.