Mortality rates for ischaemic heart disease, as estimated from death certificates, show highly significant differences between countries. In order to study the validity of mortality rates for ischaemic heart disease, the authors, involved in the MONICA project, have compared the results obtained from the conventional death certificate code with the data collected in a complementary enquiry conducted for all deaths possibly due to ischaemic heart disease. Three hundred and thirty patients, aged from 25 to 64 years, belonging to the urban community of Lille, and who died between October 1 and December 31, 1984, were included in this study. The sensitivity of the death certificate for the diagnosis of ischaemic heart disease was 77.9 percent and its specificity was 95.9 percent. The concordance rate between death certificate and complementary enquiry was not modified by age, sex, socio-professional category, family situation, place of death and doctor who signed the certificate. The complementary study proved impossible in 31.8 percent of the cases, usually because the doctor who signed the death certificate was not fully conversant with the patient's condition. Our results therefore confirm that death certificates are valid to study mortality from ischaemic heart disease.