A case of endocarditis preceded by dental treatment without prophylactic antibiotics and caused by Haemophilus parainfluenzae in a patient with a biological heart valve prosthesis is described. This bacteria grows slowly in the usual blood culture media and, because of this, the etiological diagnosis was not established until after incubation for 11 days. Blood cultures from patients suspected of having endocarditis should be incubated for longer than the usual five to seven days. Close cooperation between the clinical microbiologist and clinicians is necessary so that important findings are not overlooked. Dental treatment of patients with heart valve prostheses should always take place under antibiotic prophylaxis.