Moraxella is an aerobic, oxidase-positive, Gram-negative coccobacillus, which is rarely associated with serious and invasive infections. We describe 4 cases of Moraxella lacunata endocarditis and review 12 previously published cases of Moraxella endocarditis, including 1 further case with M. lacunata, 5 with M. catarrhalis, 2 with M. phenylperuvica and the remainder consisting of 1 case each of M. liquefaciens, M. osloensis, M. nonliquefaciens and 1 non-specified. Of these 16 patients, 5 had prosthetic valves, 5 suffered from an underlying heart abnormality, and the other 6 had normal hearts. Therapy consisted of a beta-lactam antimicrobial and, in several instances, an aminoglycoside as well. The mean duration of antibiotic treatment was 35+/-13 d. Four patients (25%) underwent surgery and 4 out of 16 (25%) died. Moraxella should be added to the growing list of organisms which may occasionally cause infective endocarditis, even in patients without preexisting valvular abnormality.