Proliferative myositis (PM) is a benign pseudosarcomatous lesion that generally affects proximal limb muscles (often the shoulder). Its unspecific signs and symptoms, rapid growth and rarity in children arouse suspicion of malignancy (e.g. rhabdomyosarcoma) in paediatric cases. We report the case of an 11-year-old girl with PM of the calf. Imaging studies ruled out bone involvement, but definitive diagnosis required histological examination of biopsy tissue, which showed small, degenerate myocytes surrounded by fibroblastic cells, cells with the appearance of ganglion cells, and fibrotically thickened endomysium. Conservative treatment was abandoned when the Achilles tendon was affected; in keeping with the literature, excision of the tumour has been followed by neither recurrence nor metastasis.