A 42-year-old man was admitted to the hospital because of pain in the left hip. On examination he was febrile at 38 degrees C and he walked with a limp. The chest, abdomen and extremities were normal. Laboratory tests showed an elevated ESR and CRP. The ANA test was positive. CT-scan of the abdomen revealed a mass in the psoas region and some dilatation of the left renal pelvis. Following the histological results of the first and second diagnostic percutaneous biopsies, the clinicians suspected idiopathic retroperitoneal fibrosis. They treated the patient with corticosteroids for a period of 4 weeks. After a short interval of improvement this treatment failed and a third biopsy was taken. Subsequently, the diagnosis of anaplastic large cell lymphoma (ALCL) was made. The patient was successfully treated with combination chemotherapy. Usually, in practice, clinical reasoning and decision-making is carried out in accordance with Bayes' theorem. But when the a priori probability of disease is unknown and the likelihood ratio of a diagnostic test unavailable, one has to combine the available 'evidence' with critical thinking, interdisciplinary communication, judgement, intuition and common sense.