Methods and compositions for treating hemophagocytic lymphohistiocytosis (HLH) are provided. The disclosure also relates generally to methods and compositions for diagnosing and treating disorders associated with elevated levels of CXCL9, elevated levels of total IFNγ, and other biomarkers. The disclosure also relates to methods of treating, delaying the progression of, or otherwise ameliorating a symptom of a disorder in patients with elevated levels of CXCL9 elevated levels of total IFNγ, and other biomarkers using agents that interfere with or otherwise antagonize interferon gamma (IFNγ) signaling, including neutralizing anti-IFNγ antibodies.