The incidence of lung cancer in females is increasing, in contrast to that seen in males. In addition, the proportion of lung cancer cases in women attributable to smoking is approximately half of that seen in males. Female sex hormones, especially estrogen, may play a key role in this. Estrogen receptors ERalpha and ERbeta have been detected on lung cancer cells and there is new evidence suggesting that hormone-replacement therapy may increase both the incidence of, and mortality from, lung cancer in women. Laboratory evidence lends credence to the carcinogenic effects of estrogens in lung cancer. This article summarizes the current evidence on their role in lung cancer.