In 10 patients (five females) suffering from multiple sclerosis with mild degree of disability, (EDSS ranging from 0 to 2) and in 10 age and sex matched control subjects we investigated lung function, respiratory muscles strength and cardiorespiratory response to incremental exercise in order to assess the metabolic cost of exercise. In the absence of any impairment of lung volumes and flows and in- and expiratory maximal mouth pressures, at peak of exercise oxygen consumption (VO2max = 1886 +/- 145 ml/min) and workload (Wmax = 137 +/- 9.8 watts) were slightly diminished in patients, as compared with controls (VO2max = 2246 +/- 196 ml/min and Wmax = 164 +/- 14.7 watts). These findings were associated with an increased heart rate (HR) and reduced oxygen pulse (VO2/HR) at the same workloads. During the whole exercise, however, the slope of the linear relationship between VO2 and work exhibited by the patients, amounting to 9.9 +/- 0.6 ml/min/watt, was similar to that of the controls (10.9 +/- 0.42 ml/min/watt). Incidentally, both at rest and during exercise, the patients showed a significantly greater minute ventilation (VE) due to a faster respiratory rate, associated with an augmented dead space (P < 0.05). We conclude that an increase of metabolic cost of exercise does not occur in multiple sclerosis patients with mild disability, suggesting a lack or a low degree of spasticity and/or ataxia elicited by the effort. Thus, their exertional capacity appears to be limited mainly by a poor training. The tachypnea observed in these patients at rest and during exercise was unexpected and the reason for adopting such a pattern of breathing is unclear.