The pathogenic bacterium Haemophilus influenzae causes meningitis, epiglottitis, pneumonia, otitis media and other infections. To further understand the genetic basis of invasive disease and to inform about the bacterium's requirements in an in vivo environment, we analysed a library of 1632 insertional Tn1545 -Delta3 transposon mutants for their capacity to cause systemic infection in an animal model. We identified 25 genes that are potentially essential for H. influenzae invasive disease, and are candidates for further exploratory research. Seven of the genes encode hypothetical proteins, the function of six of which could be tentatively assigned on the basis of functional motifs and low homology to other bacterial genes. Eleven genes encode central metabolic enzymes or transporters; eight encode proteins that interact with DNA or modify other proteins; and four encode enzymes involved in the elaboration of classical virulence determinants. Two genes have no known function. Independent mutagenesis of six of the 25 genes and determination of the competitive index confirmed that these genes are important or essential to the organism in an in vivo environment. This genome-wide analysis has identified metabolic and other genes required during invasive disease, and the findings may lead to new interventions to prevent and treat H. influenzae infections.