We developed a system for 24-hour ambulatory recording of blood oxygenation (SpO2) and electrocardiography (ECG). Using this system, we studied 10 healthy volunteers and 7 patients with chronic pulmonary diseases. The system incorporated a portable pulse oximeter (SM50) manufactured by Fukuda Denshi KK; the first and second channels were used to record ECG data and the third was used to record SpO2 data. An SpO2 sensor (Dispo-sensor D-25; Nellcor Inc.) was applied to the fourth of fifth finger. The SpO2 data (MicrO2; Siemens AG) were digitized and stored in the ambulatory recording device; the ECG was recorded simultaneously. The data were analyzed with a model DMW-9000H analyzer (Fukuda Denshi KK). A custom-designed program was also used, to remove noise errors. In the healty volunteers, SpO2 was at least 90% for the entire 24 hours. In all the patients, SpO2 fell below 90% at rest during the night or after a 15-minute walk. Transient atrial tachycardia was observed in 3 patients, and during the tachycardia the SpO2 was low. The number of extra ventricular beats divided by the total number of beats increased more in the patients than in the healthy volunteers (1.21 +/- 0.89 vs 0.6 +/- 0.3%, p < 0.05). SpO2 did not change significantly in the patients. In outpatients and in patients receiving home health care, the present system facilitates simultaneous diagnosis of respiratory failure an arrhythmias. In patients with chronic pulmonary diseases desaturation may cause transient atrial tachycardia.