Haemophilus parainfluenzae, a human commensal, is an infrequent cause of serious disease. A case of endocarditis caused by this organism in a five year old boy with complex congenital heart disease is reported. The course ofthis disease was very aggressive, leading to heart failure, disseminated intravascular coagulation and multiorgan failure in spite of appropriate antibiotics and surgical intervention. The difficulties in the detection and identification of H parainfluenzae using conventional culture based technology, and the potential role ofmolecular techniques, are highlighted. ( Clin Pathol 1997;50:72-74) Keywords: Haemophilus parainfluenzae; endocarditis.Haemophilus species are members of the normal oropharyngeal flora in healthy people. (fig 1). The isolate was resistant to amoxycillin (non-,B-lactamase mediated) but was sensitive to cefotaxime, gentamicin, ciprofloxacin, and rifampicin by Stokes method. Minimum inhibitory concentrations and tests for synergy could not be satisfactorily determined because of poor growth of the organism. Antibiotic therapy was changed to a combination of cefotaxime, rifampicin and gentamicin. However, the patient remained pyrexial and later deteriorated further, developing a coagulopathy and signs of heart failure. On day 17, he underwent emergency cardiac exploration, during which he suffered a brief period of cardiac arrest. Large vegetations were found around the dehisced VSD patch, which was excised, and the edge was debrided. There was no growth from this material, nor from several further blood cultures. Postoperatively, the patient required mechanical ventilation and inotropic support. After initially improving, his condition deteriorated again, with development of multiorgan failure. As a result of deteriorating liver and kidney functions, gentamicin and rifampicin were stopped and from day 28, antibiotic therapy was continued with cefotaxime and ciprofloxacin. Because of persistent signs of sepsis, teicoplanin, and later vanco-