The diagnosis of infectious endocarditis (IE) remains difficult. The delay until diagnosis is remarkably long. Despite progress in microbiological culture technology and the use of echocardiography, no improvement has been seen in recent years. We analyze the causes of delay in all patients with IE admitted to the Cantonal Hospital Aarau between 1976 and 1987. 60 patients with 62 episodes were included in the study. The delay from first medical consultation with IE-related symptoms to hospital admission was 46 days. In cases of antibiotic pretreatment (45%) it extended to 66 days. IE was included in the differential diagnosis by the referring physicians in 29% only, whereas it was considered in the hospital on admission in 64%. Despite long lasting fever of unknown origin, only in one case was a blood culture performed before hospitalization. The symptoms of IE were different at the beginning of the disease compared to the time of admission (general malaise in 35% and 87% respectively, fever 80% and 85%, night sweat 2% and 66%, weight loss 0% and 56%, chills 21% and 47%, joint pain 16% and 27%, dyspnea 13% and 24%). We conclude that the main reasons of the diagnostic delay in IE are antibiotic therapy prior to microbiological workup, different symptoms at the onset of IE and at the time of hospitalization, insufficient awareness of the disease among general practitioners and omission of blood cultures in patients with fever of unknown origin, especially where there is a history of valvular heart disease.