To determine the carboxyhemoglobin (CO-Hb) predictive intervals in active and passive smokers and to obtain an equation expressing the relation of CO-Hb to number of cigarettes smoked, we studied 233 outpatients referred to an urban university hospital for arterial gas measurement. Patients were excluded if they were receiving oxygen therapy or had been hospitalized in the two months before the study. The patients were classified as non smokers (57), passive smokers (54), smokers of less than 11 cigarettes (22), smokers of 11 to 20 (41) smokers of 21 to 40 (44) and smokers of over 40 (15). All patients answered a questionnaire on exposure to tobacco smoke or other sources of CO. Blood gases and co-oximetry were measured in all patients. Mean CO-Hb and 95% confidence intervals were 1.53% (0.78-1.85%) in smokers and 2.59% (1.89-3.29%) in passive smokers. The linear equation that best expressed the relationship was CO-Hb = 0.153 x number of cigarettes + 1.1 exposure to other sources (1 or 0) + 1.39 (SD 0.84)%. Hemoglobin level was significantly higher in the two groups smoking more than 21 cigarettes. We conclude that the predictive intervals is 1.9% in non smokers who are not exposed to other sources of CO. Passive smokers have significantly higher levels of CO-Hb than non smokers. Heavy smokers have polycythemia.