There is disclosed a pharmaceutical composition and method for treating sepsis, including septic shock and ARDS (acute respiratory distress syndrome), comprising administering an effective amount of a HMG1 antagonist. There is further disclosed a diagnostic method for monitoring the severity or potential lethality of sepsis or septic shock, comprising measuring the serum concentration of HMG1 in a patient exhibiting or at risk of exhibiting sepsis or septic shock symptoms. Lastly, there is disclosed a pharmaceutical composition and method for effecting weight loss or treating obesity, comprising administering an effective amount of HMG1 or a therapeutically active HMG1 fragment.