A 53-year-old man developed severe acute systemic illness three weeks after an upper respiratory tract infection. Serial blood cultures grew Moraxella catarrhalis. During antibiotic treatment, fever and infectious parameters disappeared, but severe aortic regurgitation developed. Aortic valve replacement was performed, during which extensive destruction of the aortic valve was noted. Endocarditis due to M. catarrhalis is very rare with, to our knowledge, only six cases having been reported to date. M. catarrhalis is a normal commensal of the upper respiratory tract, but in unpredictable circumstances can become an important pathogen. Bacteremia due to this organism therefore requires prompt treatment, as serious organ complications, including endocarditis, can occur.