Since the introduction of the Hib vaccine, the overall incidence of Haemophilus influenzae infection has decreased among children with a subsequent increased incidence of invasive disease in adults. A majority of invasive infections are now identified as serotype f and other nontypable strains. Carlsen et al. present a case of septic arthritis due to H.influenzae, serotype f in a patient with Waldenström macroglobulinemia. Patients with underlying immunodeficiencies are especially prone to this relatively rare phenomenon. Physicians must remain vigilant in entertaining this diagnosis in patients with immunodeficiencies so as to prevent associated morbidity and mortality.