Over the last decade, an increase in invasive infections due to Moraxella catarrhalis has been reported. We have analysed 58 cases of bacteraemia due to M. catarrhalis reported in the literature and 3 cases found in Iceland, a total of 61 cases. Patients with bacteraemia could be divided into 3 groups on the basis of host factors. They were either immunocompromised, had underlying respiratory disorders. or were normal hosts. The clinical manifestation of M. catarrhalis bacteraemic infection ranged from a mild febrile illness to a fatal disease. The severity of the clinical picture did not reflect the patients' condition at the time of bacteraemia. The port of entry of the bacteraemia was frequently not elucidated in immunocompromised patients. Patients with a contributory respiratory tract disorder were more likely to develop bacteraemia as a result of a lower respiratory tract infection, whereas bacteraemia in a normal host was more frequently due to an upper respiratory tract infection. The overall prognosis of M. catarrhalis bacteraemia was good, the exception being when it caused endocarditis (5 cases), where mortality rates as high as 80% have been reported.