AIMS: To use a sensitive test of acute myocardial damage--immunohistological detection of complement component C9--to assess the prevalence of damage in an unselected series of hearts taken at necropsy in adults. METHODS: Sections of formalin fixed and paraffin wax embedded myocardium were cut from 128 consecutive necropsy cases on which a block of heart had been taken. These were stained with an immunohistological method for C9. Necropsy findings were reviewed and clinical risk factors for myocardial damage noted. The extent of C9 immunostaining was correlated with clinical and pathological findings. RESULTS: There was immunostaining for C9 in 109 heart sections (85%). Most had conventional evidence of coronary artery disease or acute or chronic myocardial abnormality, but necrosis was identified by orthodox microscopy in only 12 (11% of C9 positive cases). In 29 cases, orthodox examination showed no abnormality, but C9 was detected. These cases had clinical risk factors for damage such as hypoxia and hypotension. Increasing age, heart weight, and total number of risk factors and pathological findings were associated with increasing extent of C9 immunostaining. CONCLUSIONS: Acute myocardial damage was common in a hospital necropsy series and its prevalence was underestimated by conventional pathological techniques. Immunostaining for C9 was a simple and useful way of detecting such damage.