Serial mild whole-body hyperthermia is a widely used balneotherapy modality for clinically inactive ankylosing spondylitis (AS) in rehabilitative medicine. Thus far, the mechanisms of its favorable influence on the symptoms of AS are not completely understood. We therefore analyzed the effect of mild whole-body hyperthermia on the systemic levels of pivotal proinflammatory cytokines. Twelve male subjects with AS and 12 healthy control subjects received nine cycles of whole-body hyperthermia (target body core temperature, 38.5°C; duration, 50 min). Serum samples were taken at the beginning of the last cycle and at 1, 6, and 24 h for measurement of tumor necrosis factor α, interleukin 1β and interleukin 6. Significant differences of cytokine levels were found between both groups. In AS patients, hyperthermia caused a significant reduction of all cytokines by 40–50%. Thus, serial mild whole-body hyperthermia in AS results in heat-induced changes of the proinflammatory cytokine network.