It is known that in normal individuals serum G-CSF levels are nondetectable. However, in neutropenic conditions the circulating endogenous G-CSF level increases significantly. In this study, we investigated serum G-CSF levels just before giving chemotherapy (CT) and al neutropenic period in 31 patients with solid tumors. Serum G-CSF levels found prior to CT were very low. These values do not give information whether the patient will enter neutropenia or not and about the degree and duration. G-CSF levels following neutropenia were significantly higher than prior values (p <.00001) and they correlated with absolute neutrophil count (ANC) (p <.005). Highest levels (1690.6+/-931.8 pg/ml) were found in patients with ANC <400/mu l (p <.05). While endogenous production was not sufficient for recovery of neutropenia, G-CSF (Filgrastim) given exogenously significantly shortened neutropenia period (p <.05).