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Tuberous sclerosis is an autosomal dominant multisystem disorder characterised by the development of benign tumours in many organs, including the brain, skin, kidneys and heart, seizures and intellectual disability. The condition results from mutations in either of two genes, TSC1 (encoding TSC1) or TSC2 (encoding TSC2). Loss of functional TSC1 or TSC2 leads to activation of mTORCl (mammalian target of rapamycin complex 1), a key regulator of multiple cellular processes including cell growth and division. Lymphangioleiomyomatosis (LAM) is a lung disorder which can lead to respiratory failure and is characterised by the proliferation of abnormal 'LAM' cells LAM occurs in patients with tuberous sclerosis and can also occur as a sporadic disorder due to acquired mutations in TSC2. Renal angiomyolipomas are benign tumours which affects80% of patients with tuberous sclerosis and 40% of patients with sporadic LAM. Sirolimus is an inhibitor of mTORCl and is used in clinical practice as an immunosuppressant. Preclinical studies suggest that TSC1 or TSC2 deficiency renders cells sensitive to mTOR inhibition. This thesis describes a phase 2 trial to assess the safety and efficacy of 2 years of treatment with sirolimus for renal angiomyolipomas in patients with tuberous sclerosis or LAM. Response of angiomyolipoma was the primary efficacy end point. Effects of sirolimus on lung function and neurocognitive function are also reported. Our data show an angiomyolipoma response rate, by RECIST criteria, of 50% in the intention to treat population. There was little change in lung function. Recall but not recognition memory tended to improve. Adverse events were common and consistent with the known toxicities of sirolimus. Our findings suggest that mTOR inhibition is a potential therapeutic strategy in the treatment of tuberous sclerosis and lymphangioleiomyomatosis.