The relationship between the abnormalities in the different routine lung function tests and the degree of functional limitation during exercise is not well established in upper airway obstruction (UAO). On the contrary, in chronic obstructive pulmonary disease (COPD), for instance, a forced expiratory volume in one second (FEV1) of 60% predicted is generally associated with a peak expiratory flow (PEF) of 60-70% pred and corresponds with a moderate physical impairment of ~20-30% [1,2]. Yet, in UAO, FEV1 will still be 70-80% pred when PEF is only 40-50% pred and it is not established what the clinical relevance is of each of these changes in terms of exercise limitation, although it is generally accepted that FEV1 is an insensitive test for UAO [3][4][5].In several respiratory disorders the maximal breathing capacity (MBC) is considered a useful index of the degree of ventilatory impairment because the exercise limitation shows a good relationship with the reduction in MBC [6,7]. In instances of ventilatory limitation the maximal exercise ventilation (V 'Emax) will often be more than 70% MBC, which is the normal ratio in healthy individuals. In addition, it has been claimed that MBC is a sensitive index of the ventilatory limitation in UAO [3,[8][9][10][11][12][13]. It seems, however, very probable that the calculated MBC (MBCc), derived from FEV1 (e.g. FEV1 ×37.5) will overestimate the actual ventilatory reserve in UAO, because of the already mentioned underestimation of the obstruction by FEV1. The relationship between FEV1 or MBCc and exercise capacity will, in addition, vary depending on the type of obstruction (i.e. fixed, variable extrathoracic or variable intrathoracic) because this will markedly influence the ratio of maximal expiratory over inspiratory flows [8]. Therefore, the measured MBC (MBCm) and not MBCc appears to be the appropriate MBC index in UAO, because it also reflects inspiratory besides expiratory flow rates, in contrast to MBCc [14][15][16][17]. The relationship between MBCm, MBCc, V 'Emax and exercise capacity is, however, not well established in UAO. Yet LINDSTEDT et al. [18] Therefore, we investigated in nine healthy subjects (age 36±9 yrs) the effects of two added resistances at the mouth (R1 = added resistance with 7.8 mm diameter; R2 = 5.7 mm) on forced expiratory volume in one second (FEV1), peak expiratory flow (PEF), airway resistance (Raw) and maximal breathing capacity (measured during 15 s = measured maximum breathing capacity (MBCm); calculated as FEV1×37.5 = calculated maximum breathing capacity (MBCc)) on the one hand, and maximum exercise capacity (W 'max), minute ventilation (V 'E) and CO 2 elimination (V 'CO 2 ) on the other.We found that R1 had almost no influence on FEV1 but decreased PEF by ~35% and increased Raw by almost 300%; it decreased W 'max by merely ~10% while maximal exercise ventilation (V 'Emax) was only 65% of control and only reached ~40% MBCc and ~70% MBCm; yet V 'E and V 'CO 2 were significantly reduced at high exercise levels indicating hypoventilat...