High mobility group box protein 1 (HMGB1) is a potent proinflammatory mediator. It has a dichotomic effect on HIV-1 replication in vitro but its role in vivo is unknown. Here we report the novel finding that plasma HMGB1 levels are elevated in HIV-1-infected patients, with the highest concentrations in patients with clinical complications. HMGB1 is likely to contribute to immunoactivation in HIV-1 infection in vivo.