The aim of this study is to assess errors in the filling-out of death certificates and difficulties for disclosing the origin of neoplasms. This study was carried out in Tijuana, Mexico. Death certificates containing a diagnosis of cancer were selected from all death certificates issued from January 1 to December 31, 1989. The following variables were identified: age, sex, place of residence and cancer site. Using these criteria, we found that 74.9 per cent of death certificates had been correctly filled out and 21.5 per cent were errors. It was seen that the main cancer sites causing mortality were the lungs, breast, cervicouterine and stomach. Cancer in unknown sites was registered in 20.5 per cent of all certificates. Implications of these results are analyzed in the light of mortality trends by causes and risk factors. We suggest that training courses are given to physicians so that they fill out the certificates properly.