Clinical findings in 36 immunosuppressed patients with lower respiratory tract infection or bacteremia with Actinobacillus hominis are described. Animal contact was only recorded for three patients; nine patients died despite appropriate antimicrobial treatment. Although infections with this microorganism seem to be rare, the fact that 37 of 46 strains characterized in this study have been found in Copenhagen indicates that under-reporting may occur. A. hominis is phenotypically relatively homogeneous but can be difficult to differentiate from other Actinobacillus species unless extensive biochemical testing is performed. Mannose-positive strains of A. hominis are especially difficult to differentiate from A. equuli. Attempts to identify A. hominis by automatic identification systems may lead to misidentifications. Ribotyping and DNA-DNA hybridization data show that A. hominis is a homogeneous species clearly separated from other species within the genus Actinobacillus.