Background: We first describe a patient who developed urosepsis from an ordinary urinary tract infection. In this case, the new hematological parameters of immature leukocytes, that is, the high-fluorescence lymphocyte cell (HFLC) and immature granulocyte (IG) counts peaked early, whereas the established infection parameters, that is, C-reactive protein (CRP) and total white blood cell count showed less dynamic regarding infection and therapy. Methods: To investigate this phenomenon in greater detail, the novel parameters HFLC and IG counts are investigated retrospectively in a cohort of 38 patients who were admitted to the anesthesia intensive care unit. Three groups of patients have been analyzed and compared: patients without signs of infection, patients with limited infections, and patients with sepsis. Data were collected with a Sysmex XE-5000 hematological analyzer. Results: In patients (n = 22) without any signs of infection, both values are very low. In patients with limited local infections (n = 10), moderate elevations of the IG and HFLC counts are seen. In patients with sepsis (n = 6), the IG and HFLC counts are significantly higher. Conclusion: The total IG count seems to be useful for distinguishing a septic patient from a nonseptic (P < 0.004). Hematological parameters have the advantage of being measured easily during routine blood cell analysis.