The study was designed to evaluate the frequency of inapparent and/or unrecorded cases of acute coronary heart disease (CHD) and its influence on overall estimates of CHD mortality and morbidity. A specially developed algorithm was used to identify unrecorded cases in official medical documentation concerning 285,736 subjects. Their fraction was found to be 20.78 and 24.48% among men and women respectively. 89.43% of them died from acute CHD. Addition of the newly identified cases to official statistics increased overall CHD morbidity rates by 26.24 and 32.41% for men and women and mortality rates by 2.13 and 2.28 times respectively. These data indicate that acute CHD is poorly diagnosed in routine clinical practice and the real CHD mortality rate is grossly underestimated.