A postmenopausal woman with severe obstructive airways disease and bronchospasm developed increased airflow limitation with the reintroduction of estrogen therapy for osteoporosis. Discontinuation of the estrogen caused symptomatic improvement and decreased her corticosteroid requirement. Readministration of estrogen caused recrudescence of her symptoms and a decline in her peak expiratory flow rate and spirometric data, which reversed with withdrawal of the estrogen therapy. Bronchospasm during the luteal phase of the menstrual cycle is well known, but exacerbation of reactive airways disease with the administration of exogenous estrogen has not previously been reported; however, with the increasing practice of reintroducing estrogen in postmenopausal women to reduce the risk of symptomatic osteoporosis, other susceptible women may suffer clinically significant deterioration of their underlying pulmonary disease.