Abstract Percutaneous needle biopsy of the vastus lateralis muscle was performed immediately prior to the intravenous injection of 5 ml of 20% fluorescein sodium, in 2 control subjects and 2 patients with polymyositis. Repeat biopsies were performed 5, 10 and 15 minutes after injection. Similar biopsies were taken prior to, and 10 minutes after, fluorescein injection in 1 control subject and 15 patients with a variety of inflammatory muscle conditions including polymyositis, some of whom were on treatment with corticosteroids. Apart from one patient, with polymyalgia rheumatica, increased penetration of dye was found only in those patients with polymyositis, particularly around areas of cellular infiltration, necrosis and phagocytosis and in the periphery of muscle fibres. Corticosteroid therapy appeared to reduce the amount of dye permeating muscle tissue in patients with polymyositis. It is suggested that reducing the abnormally increased vascular permeability in damaged muscle from patients with polymyositis may represent one mode of action of corticosteroids in this condition, and that the amount of fluorescein permeating muscle may be helpful in the diagnosis where conventional clinical and histological criteria are not conclusive.