Macrophage activation syndrome (MAS) is a frequent complication of adult-onset Still's disease (AOSD) and is characterised by organ dysfunction, cytopenia and coagulopathy. There are few data regarding the use of granulocyte colony-stimulating factor (G-CSF) as a treatment for neutropenia in MAS due to AOSD. This case describes a previously healthy mid-20s patient who was diagnosed with AOSD and subsequently MAS. Although his clinical symptoms and ferritin levels responded well to steroids, interleukin-1 inhibition and Janus kinase inhibition, he developed prolonged and profound neutropenia for which he received one dose of G-CSF. His MAS flared markedly, requiring intensive immunosuppression and significantly prolonging his hospital stay. This report illustrates that G-CSF carries the risk of worsening inflammation leading to MAS, particularly in auto-inflammatory conditions such as AOSD.