The Oxygen Status Algorithm is a computer program which uses measurements from a pH & blood gas analyser and a hemoximeter to calculate the oxygen status and the acid-base status of the arterial blood. Version 3 features on-line data collection from the analyser; storage of up to 2000 patient cases in a Lotus 123 file format; printing of a Cumulated Patient Report in addition to the Patient Status Report; combination of arterial and mixed venous data for calculation of the shunt and the oxygen consumption rate (when cardiac output is keyed in); calculation of reference values for fetal haemoglobin for newborns (when gestational age is keyed in). Examples of applications answer the following questions: 1) Does hyperventilation improve the oxygen supply to the tissues? No, for a normal person a slight hypoventilation with a pCO2 of 8.5 kPa provides a maximal oxygen extraction tension. 2) What is the optimal hyperventilation at the top of Mt. Everest (ambient pressure 33 kPa)? Hyperventilation to a pCO2 of about 1.4 kPa provides a maximal oxygen extraction tension of 2.4 kPa for an unacclimatized person. 3) Which change in haemoglobin oxygen affinity would be equivalent to a decrease in arterial pO2 to 6.3 kPa? The oxygen extraction tension would decrease to 4.0 kPa and the same value would be caused by a decrease in half-saturation tension to 2.8 kPa, a decrease which could be due to a moderate alkalaemia (pH = 7.54) combined with a moderately decreased 2,3-diphosphoglycerate concentration (3.4 mmol/L). 4) Is temperature correction of the measured pO2 and pCO2 to the actual body temperature needed? Yes, for example, omitting temperature correction even when the patient temperature is only slightly decreased to 36 degrees C would result in a negative value for the calculated arterio-venous shunt fraction when the actual value, using temperature correction, is 11%. 5) Does the alpha-stat approach of pCO2 and pH regulation in hypothermia, where pH is allowed to rise as in blood in vitro, cause a fall in mixed venous pO2 below the critical value? No, although the mixed venous pO2 will be lower than with the pH-stat approach (constant pH at body temperature), it remains above the critical mixed venous pO2 level. The program is intended for clinical routine use as well as teaching purposes. It has context sensitive help as well as an extensive help index. A number of "demo" cases are provided with annotations in a separate file.