The progression of lymphangioleiomyomatosis, a rare lung disease in women, is thought to be influenced by hormonal factors. We studied the rate of decline in FEV₁ and carbon monoxide transfer factor (Tl _CO) in a national cohort of patients with lymphangioleiomyomatosis in the United Kingdom and its relation to two factors that might influence the disease, menopausal status and progesterone treatment. We used retrospective data from hospital notes, and of the 50 patients identified 43 had suitable lung function data spanning at least 3 mo. Mean (SD) annual decline in FEV₁ was 118 (142) ml for all patients, and these figures changed little when only data spanning at least 2 and 3 yr were analyzed. There was considerable variation in the rate of decline between subjects, however, and although it tended to be less among postmenopausal women and those receiving progesterone, patient numbers were smaller and the findings were not significant. There was a significant reduction in decline in Tl _CO in premenopausal patients receiving progesterone and in both FEV₁ and Tl _CO after starting progesterone in six patients who had data before and after starting treatment. This study documents the rapid decline in lung function in lymphangioleiomyomatosis, confirms the wide variation between patients, and provides some support for the suggestion that disease progression may be reduced by progesterone. The data provide a basis for designing prospective studies of treatment for lymphangioleiomyomatosis.