Background. Corticosteroid-induced lipomatosis results from hypertrophy within adipose tissue; the condition is frequently asymptomatic and its incidence is underestimated. We report a case of mediastinal lipomatosis that is rare in terms of both site and presenting symptoms. Case report. A 46-year-old woman with no disease history other than obesity with a weight of 90 kg had been treated since 2002 for mixed connective tissue disease (profound lupus and dermatomyositis). She had been treated with oral corticosteroids (1 mg/kg/d). Two months after the start of treatment, she presented chest pains, resting dyspnea particularly aggravated in dorsal decubitus, chest edema in the subclavicular space and jugular turgescence. Chest x-ray revealed widening of all levels of the mediastinum. The chest CT scan showed lipomatosis throughout the entire mediastinum with no associated chest abnormalities or pericardial effusion. Rapid downward dosage adjustment of corticosteroids to 10 mg/d coupled with synthetic antimalarials resulted in gradual reduction of symptoms. The chest scan performed two months later short stabilization of the patient’ s mediastinal lipomatosis. Discussion. The effects of long-term of glucocorticosteroid therapy are well-known, in particular Cushing’ s syndrome. Lipomatosis has been described more recently and affects different axial regions. Mediastinal localization is seen in 15% of patients treated. This presentation is less common than orbital and epidural localizations. Although often asymptomatic, as in our own report, it may present with worrying symptoms that pose real diagnostic problems. The diagnostic examinations of choice are CT scan or MRI. Regression following discontinuation or reduction of corticosteroids is inconsistent and often gradual. Background. Corticosteroid-induced lipomatosis results from hypertrophy within adipose tissue; the condition is frequently asymptomatic and its incidence is underestimated. We report a case of mediastinal lipomatosis that is rare in terms of both site and presenting symptoms. Case report. A 46-year -old woman with no disease history other than obesity with a weight of 90 kg had been treated since 2002 for mixed connective tissue disease (profound lupus and dermatomyositis). She had been treated with oral corticosteroids (1 mg / kg / d). Two months after the start of treatment, she presented chest pains, resting dyspnea particularly aggravated in dorsal decubitus, chest edema in the subclavicular space and jugular turgescence. Chest x-ray revealed widening of all levels of the mediastinum. The chest CT scan showed lipomatosis the entire mediastinum with no associated chest abnormalities or pericardial effusion. Rapid downward dosage adjustment of corticosteroids to 10 mg / d The effects of long-term of glucocorticosteroid therapy are well-known, in particular Cushing’s syndrome. This presentation is less common than orbital and epidural localizations. Although often asymptomatic, as in our own report, it may present with with worrying symptoms that pose real diagnostic problems. The diagnostic examinations of choice are CT scan or MRI. Regression following discontinuation or reduction of corticosteroids is inconsistent and often gradual.