Purposes Heatstroke (HS) is a life-threatening condition, manifested by systemic inflammation and multiorgan failure. Rapid recognition and treatment are life saving. We report a laboratory-oriented characterization of HS by low plasma C-reactive protein (CRP) level and propose its usefulness in distinguishing this type of hyperpyrexia from central nervous system-associated high core temperature. Methods After institutional review board approval, records of patients admitted to general intensive care unit between August 2008 and September 2011 with core temperature 39.0 C or higher due to HS or meningoencephalitis (ME) were reviewed. Patients' demographics, CRP on admission and 24 to 48 hours later, serum creatinine, creatine phosphokinase, platelets count, international normalized ratio, alanine transaminase, serum pH, and lactate levels were retrieved. Results Thirty-six patients were admitted to the intensive care unit with high core temperature: 19 patients, aged 21 to 85 years, had HS; 17 individuals, aged 22 to 81 years, had ME. None of the HS individuals had infection. Twelve HS patients were previously healthy; in 13 patients, the event occurred postexercise. Mean admission CRP levels was 2.1 ± 3.3 mg/L in the HS group compared with 129 ± 84 mg/L in the ME patients (P <.0001); mean 24- to 48-hour CRP levels were 14.6 ± 16.8 vs 139 ± 98 mg/L, respectively (P <.0001). There were no clinically significant differences between the groups regarding laboratory parameters indicative of end-organ damage. Six HS patients underwent computed tomography and/or lumbar puncture before starting intensive cooling, due to misdiagnosis; 5 of them died subsequently. Conclusions Low serum CRP levels characterize non-central nervous system-associated HS. This available laboratory test could identify noninfectious hyperthermic patients upon admission, saving precious time until treatment and avoiding unnecessary diagnostic tests.