A 38 year old woman with diabetes mellitus and bronchial asthma was admitted to hospital with pneumonia caused by Mycoplasma pneumoniae; she recovered promptly on erythromycin treatment. Six weeks later she presented with aortic valve endocarditis without concurrent lung disease. A concurrent increase in titres of antibody to Legionella bozemanii, L longbeachae, and L jordanis indicated a Legionella infection. Legionella infection should be considered, even in the absence of pneumonia, in cases of endocarditis where no other cause can be detected.