The goal was to determine values of oxygen saturation from oximetry measurements over extended time periods in the context of prescribing or discontinuing supplemental oxygen. For group 1, when supplemental oxygen therapy was not in use, mean SpO2 values were significantly lower than when it was in use. Without supplemental oxygen, six of eight patients spent greater than 10 percent of the time at or below 88 percent saturation. For group 2, three of the nine patients spent 10 percent of the time at or below 88 percent SpO2. In 16 of 17 patients, lowest room air recorded values of SpO2 were less than 88 percent. We conclude that in many patients with chronic respiratory illness, prolonged monitoring of SpO2 will disclose the presence of hypoxemia. There may be substantial differences in health care cost and outcome depending upon the criteria chosen to prescribe or continue supplemental oxygen.