Neutrophilia is a hallmark of adult-onset Still's disease (AoSD). We aimed to investigate the levels of granulocyte colony-stimulating factor (G-CSF), an essential regulator of neutrophil production and function, in the pathogenesis of AoSD.Sera were collected from 70 patients with AoSD and 20 healthy controls (HCs). The levels of G-CSF were determined by ELISA. Low-density granulocytes (LDGs) were quantified by flow cytometry. Correlations between G-CSF levels and disease activity, laboratory parameters, or LDGs levels in patients with AoSD were analyzed by Spearman's correlation test.Active AoSD patients presented significantly higher levels of G-CSF compared to inactive AoSD patients (p<0.001) and HCs (p<0.0001). The levels of G-CSF were significantly decreased after active AoSD patients achieved disease remission (p=0.0015). The levels of G-CSF were significantly correlated with CRP, ESR, ferritin and systemic score in AoSD (p<0.0001). Significant correlations between the levels of G-CSF and circulating neutrophils (p<0.0001), neutrophil-to-lymphocyte ratio (p<0.0001), percentages of LDGs in the PBMCs (p=0.0042) as well as absolute numbers of circulating LDGs (p=0.0180) were identified. Patients with fever, evanescent rash, sore throat, arthralgia, myalgia, lymphadenopathy or hepatomegaly/elevated liver enzymes displayed significantly higher levels of G-CSF compared to patients without these manifestations (p<0.05).Our findings indicate that G-CSF is implicated in the pathogenesis of AoSD, and targeting G-CSF may have therapeutic potential for AoSD. In addition, introducing circulating G-CSF levels into the clinical assessment system may help to monitor disease activity.