The clinical and microbiological characteristics, the surgical indications and procedures, the evolution and the principal prognostic factors were reviewed in 46 cases of infectious endocarditis operated in the active phase. Using this date, the authors try to determine the optimal time for surgery during the acute active phase of infectious endocarditis. The study population comprised 28 men and 18 women aged 7 to 64 years (average age: 30). The patients were selected on strict criteria: positive blood cultures during the 48 hours prior to surgery (29 cases), positive valve or valve prosthesis culture (15 cases), the presence of an active cardiac abscess at surgery (7 cases), the presence of a large number of bacteria on histological examination of the valve (17 cases). The patients were divided into two groups: those with endocarditis of native valves (27 cases) and those with endocarditis on prosthetic valves (19 cases). The preoperative clinical features included all the classical signs of IE but congestive cardiac failure was particularly prevalent (62% of cases). Microbiologically, most cases of native valve endocarditis (67%) were due to sensitive organisms (streptococci) whilst the more virulent organisms (staphylococci, gram-negative bacteria and fungi) were observed in prosthetic valve endocarditis (64% of cases). The commonest surgical indication was haemodynamic deterioration (30 cases). The indications were mixed in 15 cases but only one case was operated for uncontrolled infection alone in this series. The surgical procedure was technically complex in 6 cases. Operative mortality was high (18 cases, 39%). The main cause of death was low cardiac output (13 cases).(ABSTRACT TRUNCATED AT 250 WORDS)