Moraxella (formerly Branhamella) catarrhalis is a gram-negative coccus now recognized as one of the common pathogens in respiratory infections. Documented cases of bacteremic pneumonia due to this organism, however, have been a rarity. Two cases of Moraxella catarrhalis bacteremic pneumonia in immunosuppressed adult patients are reported. The clinical characteristics of these patients together with those of the seven adult and the six pediatric patients reported to date in the literature, are analyzed. All patients had an underlying condition and most were male. The mean age was 64.9 years. No adult patient had skin lesion, although purpuric rash was frequent in children. The overall morality rate was only 13.3%, in spite of the underlying diseases. In three patients the pneumonia was nosocomial. The seasonal recovery of Moraxella catarrhalis in respiratory infections is significantly increased during the late fall through early spring period. Because most strains are beta-lactamase positive, empiric use of penicillin, ampicillin or amoxicillin for this organism can no longer be recommended.