We developed a new method of evaluating the tolerance for physical exercise in patients with chronic respiratory disease. Using a newly developed portable pulse oxymeter, with which we could measure kinetic energy (physical activity), calculated from the vertical acceleration involved in physical movements in the patient's daily life, we considered the correlation between the characteristics of the distribution of oxygen saturation (Spo₂) and the degree of physical activity. The characteristics of Spo₂ distribution in normal healthy subjects are uniform at all degrees of physical activity. In patients with chronic respiratory disease who complained of dyspnea on exertion, these characteristics became more uneven as physical activity increased. By comparing the characteristics of Spo₂ distributions at rest with those at certain degrees of physical activity, we could quantitatively evaluate the exercise tolerance of patients with chronic respiratory disease, while monitoring their physical activity and Spo₂ in daily life, without burdening the patients with stress such as would be imposed by the treadmill test. This new method is applicable for determining the indications for home oxygen therapy. Its application in home health care could offer a useful evaluation of a patient's activities of daily living and also early discovery of aggravation of chronic respiratory failure.