BACKGROUND Deaths are coded to the International Classification of Diseases (ICD) category, 'Symptoms, Signs, and ill-defined Conditions' when there is insufficient information for cause of death determination. Due to difficulties of diagnosis of coronary heart disease (CHD) death and since CHD is the leading cause of death among US adults, CHD is the most likely cause of ill-defined deaths. METHODS Vital statistics and census data were used to create annual age-adjusted ill-defined rates, unrevised CHD rates, and CHD rates revised to include ill-defined deaths for US African Americans and whites, ages 35-74 years, during 1968-1988. Ill-defined and CHD mortality trend analyses were conducted. RESULTS In 1968, African American/white ratios of ill-defined mortality were 5 among men and 7 among women; following steep declines in ill-defined mortality among African Americans, ratios were 3 among men and 2 among women in 1988. In 1968, approximately 3% and 1% of all deaths among African Americans and whites, respectively, were certified to ill-defined causes; in 1988, approximately 1.5% of deaths among African Americans were coded ill-defined, with no change among whites. Revised CHD rates showed substantially higher excess CHD mortality among African Americans than whites compared to unrevised CHD rates. Declines in revised CHD mortality steepened throughout the study period among men, and among women began to decelerate after 1978. CONCLUSIONS Ill-defined mortality was of sufficient magnitude to potentially contribute to substantial underestimation of racial disparities in CHD mortality. Also, temporal changes in ill-defined mortality may have affected CHD trends which are used to evaluate the efficacy of public health interventions.