From 1976 to 1985, 27 adult invasive Haemophilus infections were observed at the University Medical Center in Lausanne. Only 5 cases (19%) were caused by Haemophilus influenzae type b, while 12 cases (44%) were due to Haemophilus species other than H. influenzae. Two out of 24 strains tested were ampicillin-resistant. The infections were meningitis in 8, pneumonia in 7, endocarditis in 5, sepsis of unknown origin in 4, epiglottitis in 2, and one gynecological infection. Except for the latter three patients, each case was associated with one or more underlying conditions. Seven patients died (26%), in three of whom death was directly related to the infectious process. This report and a review of the literature show that adult invasive Haemophilus infections are not uncommon and may be serious. Associated underlying diseases and advanced age are generally present. In contrast to infections occurring in children, invasive Haemophilus infections in adults are not restricted to encapsulated Haemophilus influenzae type b strains.