['Hantaviruses are important human pathogens that cause clinical diseases characterized by renal and cardiopulmonary manifestations. Their pathogenesis is currently poorly understood. We have studied the role of the complement system in the pathogenesis of Puumala (PUUV) hantavirus infection.', 'We studied the activation of complement by measuring the terminal complement complex SC5b-9 and complement component C3 and C4 levels in patients with acute PUUV infection. Several laboratory parameters and clinical findings reflecting the severity of PUUV-HFRS were evaluated with regard to complement activation.', 'The levels of SC5b-9 were significantly increased and C3 decreased in the acute stage as compared to the levels at full recovery (P < 0.001). We found that SC5b-9 levels were higher in patients with chest X-ray abnormalities than in patients with a normal X-ray during the acute stage (P = 0.028). Furthermore, SC5b-9 and C3 levels showed significant correlation with several clinical and laboratory parameters that reflect the severity of the acute PUUV infection.'] Does complement activation in Puumala hantavirus infection correlate with disease severity?