Serum granulocyte‐colony stimulating factor (G‐CSF) was measured with an ELISA method in patients with acute bacterial and viral infections, or with and atypical pneumonia. Before initiation of antibiotic treatment, G‐CSF was found to be significantly increased (799 ± 1501 ng/1) in sera from 34 patients with and acute bacterial infection compared with the 27 patients with the 27 patients with viral infection (58 ± 34 ng/1; P < 0.001) and with the eight patients with an atypical pneumonia (60 ± 33) ng/1; P < 0.001). No significant difference in G‐CSF levels was seen between gram‐positive and gram‐negative bacterial infections. In septic shock, increased G‐CSF levels were seen both in patients with leucocytosis and leucopenia. In uncomplicated bacterial infections, both G‐CSF and IL‐6 were increased on day 0, and decreased rapidly after initiation of antibacterial therapy and before the patients became afebrile. In bacterial infections on day 0, G‐CSF levels correlated with mononuclear cells (r_s=−0.62, p < 0.001), IL‐6 (r_s= 0.40, P < 0.05 and S‐MPO (r_s=−0.5, P < 0.01). In viral infections, G‐CSF was correlated with mononuclear cells (r_s= 0.041, P < 0.05), White blood cell counts (r_s= 0.56, P < 0.01), neutorphils (r_s= 0.41, P < 0.05) and CRP (r_s= 0.47, P < 0.05). We conclude that G‐CSF is rapidly rised in the blood in acute baterica infections but not in acute viral infections or in infections with Mycoplasma pneumonia. Our results also support the theory that G‐CSF is involved in the mechanisms of mobilization of neutrophils into the peripheral circulation.