The histopathological diagnosis of the child tumour group formed by small dark cells is frequently difficult to establish. Sometimes the correct diagnosis can not be determined before various imunohistochemical, cytogenetic and molecular genetic techniques have been used. Paediatric non-Hodgkin’s lymphomas belong to this group of tumours. The diagnostic mistake in a ten-years-old boy, previously treated for aggressive fibromatosis for two years, is described. Surprisingly the fourth biopsy proved a quite unusual sclerotic variant of malignant B-cell lymphoma. Modified treatment followed by autologous bone marrow transplantation led to complete remission already persisting for six years. By coincidence at the same time the patient’s father underwent bone marrow transplantation because of recurrent malignant lymphogranuloma.

        Key words: diagnosis, children, non-Hodgkin lymphoma, genetic predisposition