A case of so-called congenital fibro(leio)myosarcoma of the small intestine in a 18-day-old female baby, treated only with surgical resection, was studied by immunohistochemistry and electron microscopy in order to investigate the proliferating cell type. The tumour cells showed positivity only for vimentin and CD 34 and were negative for smooth muscle actin, desmin, alpha-sarcomeric actin, factor VIIIR: Ag and S-100 protein. Ultrastructural findings showed oval nuclei with prominent nucleoli, rare intracytoplasmic mitochondria and well developed rough endoplasmic reticulum. According to histoimmunological and electron microscopy findings the proliferating cells were likely to be of fibroblastic origin. A 7-year follow-up showed a favourable clinical evolution thus confirming that surgical resection can be a sufficient therapeutic approach. The morphological findings and clinical behaviour suggest that more appropriate terminology for this tumour would be "aggressive congenital fibromatosis" which better highlights its local progressive invasion without metastases.