['Some patients with chronic inflammatory diseases such as rheumatoid arthritis (RA) improve rapidly from anti-tumor necrosis factor (anti-TNF) therapy. No sensitive markers are available that might predict outcome of anti-TNF therapy. We undertook this study to investigate the predictive value of hypothalamic-pituitary-adrenal (HPA) axis hormones for clinical improvement during anti-TNF therapy.', 'An observational study in 23 RA patients was followed by a validation study in 38 RA patients. The patients receiving anti-TNF antibodies had no glucocorticoid treatment, and we measured baseline serum levels of adrenocorticotropic hormone (ACTH) and cortisol. Improvement during anti-TNF antibody treatment was judged by the Disease Activity Score in 28 joints (DAS28), and serum levels of cortisol were measured at followup.', 'The observational study demonstrated that improvement in the DAS28 correlated negatively with baseline serum levels of cortisol (R=-0.520, P=0.011) and the cortisol:ACTH ratio (R=-0.700, P=0.0002). In the longitudinal part of the study at followup, those patients with good improvement and initially low serum levels of cortisol demonstrated an increase of serum cortisol, in contrast to patients with little or no improvement. Findings in the observational study were supported by those in the validation study in a group of RA patients with less inflammation (correlation of improvement in the DAS28 with cortisol:ACTH ratio: R=-0.320, P=0.025).'] Does increased cortisol relative to adrenocorticotropic hormone predict improvement during anti-tumor necrosis factor therapy in rheumatoid arthritis?