The morning stiffness and pain of rheumatoid arthritis (RA) is accompanied by a rise in serum interleukin-6 (IL-6) from 2 am to 7 am. Using a formulation that releases prednisone at 2 am (after ingestion at 10 pm), we studied the circadian dynamics of serum IL-6, other cytokines, and cortisol in 9 patients before and after 2 weeks, therapy. Significant improvements occurred in morning stiffness, pain, disease activity, and the acute-phase response. Only IL-6 showed measurable cytokine circadian variation, its high pretreatment peak was abolished, and changes in IL-6 correlated with the changes in morning stiffness. Following treatment, afternoon and evening serum cortisol was reduced, but in the early morning cortisol peak concentration increased. Thus the severity of morning symptoms is related to nocturnal serum IL-6 concentration. The specific timing of the medication, linked to the interaction between IL-6 and the hypothalamic-pituitary-adrenal (HPA) axis, may correct a postulated deficiency in HPA control in RA.