Twenty healthy hospital workers produced maximal expiratory flow-volume curves before and after three vital capacity inhalations of an 80 percent helium and 20 percent oxygen mixture (HE+O2) in the morning and afternoon for four days during one week. Ten healthy trade union apprentices underwent the same tests, twice on one day and again one month later. Measurements made from curves (and their mean coefficients of variation) were: VisoV (105 percent) FVC (3 percent), FEF50% (6 percent), FEF75% (8 percent), the ratio of FEF50% breathing He+O2 to FEF50% breathing air (6 percent), and the ratio of FEF75% breathing He+O2 to FEF75% breathing air (9 percent). Differences among separate observers contributed significantly to the high variability of the VisoV. Diurnal changes and training effects over the week of study were not significant. In conclusion, VisoV is poorly reproducible compared with the FVC and expiratory flow rates at low lung volumes breathing air and He+O2. This should be considered when interpreting results in a given individual.