To examine the level of muscular work and ventilatory response to stair climbing (mobile Stairmaster staircase, 1 step/2.5 s), respiratory gas exchange, ventilation, heart rate and arterial pressure were monitored in patients with chronic heart failure and their response compared with that of normal individuals. Aerobic capacity (maximal oxygen uptake) and anaerobic threshold during treadmill exercise were also determined and used to ascertain the metabolic cost of stair climbing. No differences were observed in the response of mean arterial blood pressure between the 12 patients and 10 normal subjects during exercise or recovery. However, the heart rate and oxygen consumption obtained during exercise were significantly lower in the patients with chronic heart failure than in the normal subjects (p less than 0.05). All patients with a maximal oxygen uptake less than 20 ml/min/kg during treadmill exercise had an oxygen uptake during stair climbing that exceeded their anaerobic threshold and, consequently, they attained a significantly (p less than 0.01) higher level of ventilation during exercise and recovery. This was not the case for those patients with greater aerobic capacity. Therefore, it is concluded that stair climbing for the 12 patients with heart failure and moderate to moderately severe impairment in aerobic capacity represents strenuous anaerobic exercise. The resultant excess ventilation may explain the limiting sensation of dyspnea that is frequently experienced by these patients during and after stair climbing.