Generally, laboratory tests, with the exception of serology and culture examinations, are not useful for the specific diagnosis of influenza. Leukocyte counts are variable according to stage; frequently low during the early stages of illness, and later become normal or slightly elevated (15) . In the present series, there were no patients with elevated WBC counts and 67% of patients showed lymphocytopenia. These findings are consistent with those found in previous studies that dealt with unspecified populations (3, 5) . One additional diagnostic laboratory feature we were able to find was an elevated CRP level while both the ESR and WBC counts remained within a normal range in all patients in our study with pandemic Korean J Radiol 11(4), Jul/Aug 2010 - (14) . Further detailed comparative studies with a larger number of patients would be required to fully explore this issue. Most patients in our study did not show a severe hospital course, although one patient's symptoms aggravated into ARDS and needed mechanical ventilation before laboratory tests revealed that he had the pandemic H1N1 2009 influenza virus infection. We considered the possibility that co-infection of the pandemic H1N1 2009 influenza virus with other unknown respiratory pathogens under the leukocytopenia status could have attributed to this symptom aggravation (9, 10), particularly as he was a soldier serving in a different unit from the majority of other patients. In addition, the symptom aggravation had occurred after he had been in contact with a patient infected with the pandemic H1N1 2009 influenza virus and there were no pandemic H1N1 2009 influenza virus cases in the unit the patient had been enrolled in. However, since his respiratory symptoms and high fever did not respond to other previous antibiotic treatment, there is still the possibility that the disease course in this case could have been solely due to a severe manifestation of the pandemic H1N1 2009 influenza virus infection.