The objective was to perform a needs assessment of trainee doctors' competences within lumbar puncture and bone marrow aspiration in paediatric oncology. Audit of 30 medical records revealed that five procedures, i.e. collegial, supervised administration of chemotherapy, examination of the child, length of bone marrow biopsy, erythrocytes in cerebrospinal fluid, and documentation of procedures in the medical record, were only handled as intended in half of the cases. Education and supervision of trainee doctors should be prioritized in order to increase their competences and decrease the risk of critical incidents.