A 75-year-old woman was admitted with a 3-month history of intermittent fever, night sweats, weight loss and dyspnoea. She was referred from another hospital where the problem had been investigated in depth without success. Whole body computed tomography scanning had shown mild splenomegaly. Results of transthoracic echocardiography, gastroscopy, small bowel biopsy, temporal artery biopsy, bone marrow aspirate and biopsy and liver biopsy were negative. Whole-body positron emission tomography was normal. Laboratory investigations showed anaemia (haemoglobin concentration 86 g/l), lymphopenia (lymphocytes 0·47 × 109/l), low platelet count (66 × 109/l) and C-reactive protein of 110 mg/l. Serum ferritin and lactate dehydrogenase levels were 2061 μg/l and 1125 iu/l respectively. Clinical examination showed abdominal subcutaneous oedema and diffuse telangiectasia of recent onset (image). A large skin biopsy revealed intravascular large B-cell lymphoma. This is a rare non-Hodgkin lymphoma characterized by the selective growth of lymphoma cells within the lumina of vessels. Its diagnosis is extremely difficult and is often made post-mortem. Clinicians should be alert to atypical cutaneous lesions, especially telangiectasia, in cases of pyrexia of unknown origin.