Neutropenia is a severe adverse-event of chemotherapeutics. Neutrophils (ANC) are mainly regulated by granulocyte colony stimulating factor (G-CSF). The aim was to characterize the dynamics between endogenous G-CSF and ANC over time following chemotherapy. Endogenous G-CSF and ANC were monitored in forty-nine breast cancer patients treated with sequential adjuvant 5-fluorouracil-epirubicin-cyclophosphamide and docetaxel. During treatment courses ANC was transiently decreased and was reflected in an endogenous G-CSF increase, which was well described by a semi-mechanistic model including control mechanisms; when G-CSF concentrations increased the proliferation rate increased and the bone maturation time reduced for ANC. Subsequently, ANC in the circulation increased leading to increased elimination of G-CSF. Additionally, a non-specific elimination for G-CSF was quantified. The ANC-dependent elimination contributed to 97% at baseline and 49% at an ANC of 0.1 center dot 10(9)/L to the total G-CSF elimination. The integrated G-CSF-myelosuppression model captured the initial rise in endogenous G-CSF following chemotherapy-induced neutropenia and the return to baseline of G-CSF and ANC. The model supported the self-regulatory properties of the system and may be a useful tool for further characterization of the biological system and in optimization of chemotherapy treatment.