Of the death certificates issued in Sweden in 1978 and stating cancer as the underlying or contributory cause of death, 1634 cases were unrecorded in the national cancer register. In 62 per cent of the cases the criteria for cancer registration were fulfilled. The non-reported cases represented a total deficit of 4.5 per cent calculated on cancer deaths in 1978. The factors responsible for the deficit were investigated. When the diagnosis had been histologically/cytologically confirmed the deficit was less than 2 per cent but was about 30 per cent when the diagnostic basis was only clinical. More than half of the non-notified cancer patients were older than 75 years. Exclusion of this age group and of myeloma and leukaemia cases gave a cancer-register deficit of 2.3 per cent. Non-notification to the Swedish cancer registry can be diminished by supplementation with data from death certificates, as practised in other Nordic countries. On regional basis these death certificates will now be collected and used as a supplement to the cancer notification in Sweden.