Current data show that women now suffer from COPD at least as commonly as men. They seem to be more predisposed to suffer the adverse respiratory consequences of tobacco smoking with the development of COPD at an earlier age and with a greater degree of lung function impairment for a given amount of tobacco exposure. This may be explained, in part, by women's greater airway responsiveness to exogenous stimuli--an increased responsiveness that is explained largely by differences in lung size and geometry. Hormonal factors and immunologic factors that influence the lung also may differ between the sexes but their significance is poorly understood. Despite the increasing prevalence of COPD among women, their disease is more likely to be overlooked as compared with the same disease in men. This gender bias in diagnosis is not likely to be remedied until physicians begin to use spirometric testing to screen for this common disease. Women who smoke may have greater difficulty quitting than men. There are documented differences in health care use between men and women who have COPD, but too few studies have been done to allow conclusions to be drawn about the impact of sex and gender on the prognosis of the disease.