Objectives: The objective was to determine the current epidemiology of infective endocarditis.

Patients and Methods: All microbiologically positive episodes of infective endocarditis treated at The University Hospital of Wales over a 9-year period from March 1987 to March 1996 was reviewed. Patients originated from the catchment area of The University Hospital of Wales or were referred from other hospitals in Wales. Data extraction was from records held in the Microbiology Department and, whenever possible, from patients' casenotes. The epidemiological parameters were: (1) age and sex of patients; (2) distribution of affected sites; (3) frequency of predisposing risk factors (cardiac and extracardiac); (4) incidence of early prosthetic valve endocarditis; and (5) mortality rates.

Results: There were 128 microbiologically positive episodes of endocarditis in 125 patients. The mean age of the population was 53.1 years and the aortic valve was the most frequently involved site of infection (51.6%). A presumed source of infection was identified in 20% if episodes. The commonest predisposing cardiac risk factor in native valve episodes was bicuspid aortic valve (16.7%) but there was no identifiable cardiac risk factor in a much larger proportion (37.7%) of native valve episodes. There was a low incidence (0.6%) of culture positive early prosthetic valve episodes and low mortality rates for both native and prosthetic valve endocarditis (12.3% and 24.5%) in this study. Viridans streptococci were the predominant organisms. In prosthetic valve episodes with onset after the 60^th postoperative day but within one postoperative year the identity of the isolate suggested, in most cases, perioperative valve contamination.

Conclusions: The epidemiology of infective endocarditis has undergone significant change. Inability to detect clinically common predisposing lesions, and the frequent absence of any identifiable predisposing cardiac risk factor mean that initial diagnosis is often difficult and demands a high index of suspicion. There was a low incidence of culture positive early prosthetic valve episodes and there were low mortality rates for both native and prosthetic valve endocarditis; these figures suggest improvements in cardiac care. The microbiological evidence indicates that the duration of the postoperative time period used for classifying prosthetic valve endocarditis into ‘early’ and ‘late’ episodes should be extended from 60 days to 1 year.