We present a case of a 16-year-old boy with a painful swelling in the popliteal fossa, slight fever, articular and muscle pain of 3 weeks duration. Plain radiography and MR examination were performed in order to characterize the lesion. Additionally to the poorly defined mass, 2 satellite nodules were shown. Subsequent biopsy revealed a granulomatous process and 2 satellite lymphadenopathies highly suggestive of cat-scratch disease. This case shows that the differential diagnosis of a soft tissue mass of the knee should not only include tumoral processes or pseudotumoral lesions, but also granulomas from infectious or non-infectious origin. Knowledge of the patients contact with cats and the detection of skin scratches is usually the clue to the correct diagnosis.