A study of undernotification to the Stockholm Regional Cancer Registry comprised cancer diagnosed in 1978. Non-notified cases were identified by linking the cancer register with two independent sources of information viz. the Swedish cause-of-death register (fatal cases) and the regional in-patient care register (non-fatal cases). The estimated deficit in the cancer register for 1978 was 4% of the total cancer incidence for that year. The unreported non-fatal cases were followed up for five years, during which time more than half of the cancer diagnoses were entered on death certificates. If the cancer register had been supplemented with information from death certificates, therefore, only about 1% of all non-notified cases would have remained unregistered five years after diagnosis.