To clarify the physiologic roles of granulocyte colony-stimulating factor (G-CSF) in infectious states in vivo, we examined the serum levels of G-CSF in patients with infection. Serum samples from 24 patients in the acute stage of infection (14 men and 10 women, age 65 to 101, without hematologic disorders), as well as samples from 32 age- matched normal elderly volunteers were investigated. Sixteen of the initial 24 patients were reexamined after the recovery phase. G-CSF levels were examined by quantitative enzyme immunoassay. The G-CSF level in normal elderly controls, 25.3 +/- 19.7 pg/mL, was not different from that reported in other findings. There was no statistically significant relationship between their G-CSF level and peripheral white blood cell count or neutrophilic granulocyte count. The G-CSF level in the acute stage of infection was 731.8 +/- 895.0 pg/mL, with a range of 30 to 3,199 pg/mL. There was no significant difference in G-CSF levels between patients with respiratory tract infection and those with urinary tract infection. In all 16 cases examined, the serum G-CSF level in the acute stage of infection was significantly higher than that after recovery phase, the latter being the same as the level in normal elderly controls. G-CSF must therefore play a significant role in human infectious states in vivo.