The actual condition of carbon monoxide content in expired air and its relation to life-environmental and physical factors was investigated in 277 pregnant women. Subjects residing in a midtown area of Tokyo have average CO concentrations in the expired air slightly higher than those previously reported. Seventy two women (26%) have CO concentrations over 10 ppM. The concentration of CO in expired air correlates with the CO level in the immediate life environment of the individual. Pregnant women with a history of abnormal gestation and/or abnormal delivery are more frequent in the areas with those exhibiting higher CO concentrations in expired air, compared to regions with women showing lower CO levels.