The authors report their experience on etiological and clinical aspects of infective endocarditis (IE). A series of 182 consecutive patients, including 83 cases of medical IE, 73 cases of IE in intravenous drug abusers (DA), 22 cases of IE on late prosthetic valves and 4 cases of IE on early prosthetic valves were evaluated since 1976. Medical IE occurred frequently in the elderly patients and in most of the cases (80%) involved natural valves with underlying abnormalities, either rheumatic (42%) or degenerative (33%) or malphormative (25%). Pre-existing valvular pathology was not needed for IE in DA, occurring in 13%, mainly due to a previous IE. In most of the cases IE in DA was a staphylococcal IE (80%) and a right-sided IE (77%). Streptococci were frequent agent both in medical and late prosthetic valves IE (55%): however, a wide pattern of microorganisms, including "unusual" pathogens like nutritionally variant Streptococci, Haemophylus parainfluenzae, Haemophylus paraphrophylus, Coxiella burnetii and the so-called "non pathogen microorganisms" (e.g. Neisseria sicca) was identified as etiological agent. As regards the clinical approach and diagnosis, the Authors underline some atypical clinical presentations of IE: the pulmonary presentation, occurring in right-sided IE, mainly in DA; the neurological presentation, suggesting staphylococcal etiology and left-sided IE; the vasculitis presentation, miming connective tissue diseases; the cardiac presentation, observed in aortic localization (1 case). One or more severe complications occurred in 65% of the patients, contributing to adverse outcomes.(ABSTRACT TRUNCATED AT 250 WORDS)