Rationale: Angiogenesis is a defining pathologic feature of airway remodeling and contributes to asthma severity. Women experience changes in asthma control over the menstrual cycle, a time when vessels routinely form and regress under the control of angiogenic factors. One vital function modulated over the menstrual cycle in healthy women is gas transfer, and this has been related to angiogenesis and cyclic expansion of the pulmonary vascular bed. Objectives: We hypothesized that changes in gas transfer and the pulmonary vascular bed occur in asthmatic women over the menstrual cycle and are associated with worsening airflow obstruction. Methods: Twenty three women, thirteen asthmatics and ten healthy controls were evaluated over the menstrual cycle with weekly measures of spirometry, gas transfer, nitric oxide, hemoglobin, factors affecting hemoglobin binding affinity, and pro-angiogenic factors. Measurements and Results: Airflow and lung diffusing capacity varied over the menstrual cycle with peak levels during menses that subsequently declined to nadir in early luteal phase. In contrast to healthy women, changes in lung diffusing capacity (DLCO) were associated with changes in membrane diffusing capacity and DLCO was not related to pro-angiogenic factors. DLCO was not different between the two groups, though methemoglobin and carboxyhemoglobin were higher in asthmatics than healthy women. Conclusion: Asthmatic women experience cyclic changes in airflow as well as gas transfer and membrane diffusing capacity supportive of a hormonal effect on lung function.