Carboxyhemoglobin levels (COHb) were determined in residents of two Mexican cities (Mexico City and Toluca) at altitudes of 2200 and 2600 meters above sea level. The figures in non-smokers of both cities were slightly higher than those seen at sea level by others: 1.63% in Mexico City and 1.54% in Toluca versus 0.5-1% at sea level. There were no intercity differences in smokers and non-smokers, but there were intergroup differences in both cities (p less than 0.01 in Mexico City, and p less than 0.05 in Toluca). Our findings suggest that the most important source of carbon monoxide in smokers is tobacco smoking, and that the COHb level may be reliable index to establish active smoking.