Lymphangioleiomyomatosis (LAM) is characterised by lung cysts and airflow obstruction. Matrix metalloproteinases have been implicated in lung destruction in LAM. We performed a randomised, double-blind trial, comparing the matrix metalloproteinases inhibitor doxycycline with placebo on the progression of LAM. 23 females with LAM were randomised to doxycycline 100 mg daily for 3 months followed by 200 mg daily for 21 months, or matched placebo. Lung function, exercise capacity, quality of life and matrix metalloproteinases levels were measured. 21 patients completed 6 months of treatment, 17 completed 1 year of treatment and 15 completed 2 years of treatment. Eight withdrew from the trial due, four due to a pneumothorax and four because of other reasons. The mean±sd decline in FEV1, the primary endpoint, did not differ between the groups being -90±154 mL·year(-1) in the placebo group and -123±246 mL·year(-1) in the doxycycline group (difference -32.5, 95% CI -213-148; p=0.35). Doxycycline had no effect upon vital capacity, gas transfer, shuttle walk distance or quality of life. Urine matrix metalloproteinases-9 measurements were lower with doxycycline treatment (p=0.03). Although with limited numbers we cannot completely exclude an effect of doxycycline, the lack of effect on any outcome makes it unlikely that doxycycline has a useful effect in LAM.