The incidence and mortality of malignant diseases show constant increase worldwide. Proper epidemiological data may establish the planning and development of oncological network, which is provided by population-based registries (in Hungary: National Cancer Registry). The quality of the reported data determines the reliability of the Registry. Recorded medical codes during everyday physician-patient encounters are part of the official documentation as well as a permanent imprint of the medical activity in the Registry's database. Uncritical coding degrades the quality of epidemiological data, moreover, leads to unnecessary patient stigmatization, which may be the base of legal procedure against the physician who authenticated the false code. However, neither graduate nor postgraduate medical training focus on coding. In addition, hospitals apply obsolete versions of coding systems which does not follow developments in medicine. The aim of the present review is presentation of proper coding in oncology, which may contribute to avoid that kind of basic professional pitfalls, and improve quality of medical activity.