Alaskan Eskimos have the highest known prevalence of invasive Haemophilus influenzae type b (Hib) disease, primarily meningitis, affecting 1-5% of all children in the first two years of life. In this population a polymorphic genetic variant of the pyrimidine pathway enzyme, uridine monophosphate kinase-3 (UMPK-3), was found to be positively associated with invasive Hib disease (relative risk 3.3) and a tendency towards a younger age at onset of illness. There was no difference in levels of naturally acquired Hib anticapsular antibody between persons with Hib disease and healthy controls in this population. This suggests that UMPK-3 may have a role in mediating non-humoral immunity to Hib. However, unlike other enzyme variants in the nucleoside synthesis pathways which result in syndromes of severe immunodeficiency, this gene appears to confer a more subtle disease susceptibility.