We will perform a 2 year double blind, placebo controlled trial of doxycycline in 40 patients with LAM. The main endpoints will be change in FEV1, other measures of efficacy, safety and dose required to suppress MMP activity. After clinical evaluation, lung function, shuttle walk, QoL assessment, blood tests, serum and urine MMPs (termed full assessment) plus baseline chest X-ray patients will be randomised to doxycycline 50 mg od or placebo. Patients will be assessed at 2 weeks for a safety screen, every 3 months for clinical evaluation and spirometry and at 12 and 24 months for full assessment. At 3 months urine zymography will be performed to see if MMPs are present in urine at the prescribed dose. Doxycycline will be increased to 100 mg bd at three months after urine zymography has been performed. To avoid withholding treatment from those who decline rapidly, patients who, on two occasions, have either a fall from baseline FEV1 of 300 ml or fall in resting SaO2 of 3% will be assessed by an independent expert (AET). Patients receiving placebo will be given the option of doxycycline according to protocol. Those receiving doxycycline the option of continuing in the study or withdrawal. These patients and those withdrawn due to recurrent pneumothorax, increase in chylous effusion or bleeding angiomyolipoma will be included in a composite safety endpoint and analysed on an intention to treat basis. Power calculations based on retrospective cohorts(Johnson and Tattersfield 1999) show that 20 patients per group will give 80% power to detect a 70 ml/year difference in FEV1 based an assumed SD for a fall in FEV1 of 75 ml/year. The mean slope of regression lines for FEV1 and FVC, plus change DLCO, shuttle walk distance and QoL in the doxycycline and placebo groups will be compared by parametric or non-parametric analysis dependent on data, time to composite safety endpoint by Caplan-Meier analysis, complications and adverse events by Chi Square test.