Circadian variation of temperature, both normal and febrile, is a well known fact. Mediators of fever are also regulators of acute phase response and are associated with stimulation of pituitary hormone production related with defervescence and with a circadian pattern of secretion. Acute phase response may consequently have circadian variations in its components. Measurements of temperature, erythrocyte sedimentation rate, complete blood cell count, serum cortisol and fibrinogen were made at 7:00, 15:00, and 23:00 h during two consecutive days in 35 patients with fever and acute infection, 15 patients with clinically active ankylosing spondylitis without fever and 10 healthy volunteers. Temperature curves showed statistically significant circadian rhythms, with higher values at night and lower ones during early morning. Erythrocyte sedimentation rate, leukocyte count and fibrinogen also showed statistically significant circadian rhythms, but with higher values at 15:00 h. Serum cortisol also showed statistically significant circadian rhythmicity but with a higher rhythm adjusted mean (MESOR), and a 100 degrees (6 h) phase shift in patients with fever, as compared to patients with ankylosing spondylitis and healthy controls. In conclusion, components of acute phase response, including fever, have circadian rhythmicity, but asynchronically. Differences between fever and ankylosing spondylitis can be due to the intensity of acute phase response stimulation.