A case of proliferative myositis, an unusual inflammatory pseudotumour of skeletal muscle, is reported. This case was particular in that the lesion was located in the popliteal fossa, developed over a long period and above all, was revealed by compression of the sciatic nerve. The diagnosis was confirmed by peripheral biopsy of the mass which showed normal muscular tissue associated with numerous fibroblasts and with characteristic basophilic giant cells. The clinical, histological and pathogenetic features of the disease are discussed with emphasis on the importance of a thorough pathological examination to exclude with certainty a possible sarcoma.