Data from an Auckland coronary heart disease register have been used to assess the validity of Maori mortality statistics produced by the National Health Statistics Centre. During the period 1983-4, 804 people aged 35-64 years and resident in the Auckland statistical region, were identified by both the register and death registration data as having died of coronary heart disease. The coronary heart disease register failed to classify the ethnicity of thirteen of these people. Of the remaining 791 cases, the register classified 80 as Maori while only 44 were classified as Maori in the national death registration data; over the period 1983-4 Maori mortality due to coronary heart disease in the Auckland statistical region was understated by 82% (80-44/44). Although some of this discrepancy may be due to differences in classification of ethnicity, the major reason for the understatement is missing information on the death registration form. Simple changes in the documentation of ethnicity could markedly reduce the degree of underreporting.