Twenty patients with fibromyalgia syndrome and 20 patients with rheumatoid arthritis (RA) were assessed as outpatients over a 3 day period with respect to peak and trough levels of plasma cortisol, growth hormone, prolactin, ACTH and thyroid stimulating hormone. Patients with fibromyalgia syndrome had loss of diurnal variation in plasma cortisol (trough levels 347.3 +/- 254.7 vs 232.8 +/- 70.0 nmol/l, p less than 0.001) compared with RA patients. Thirty-five percent (7/20) of patients with fibromyalgia syndrome and only 5 percent (1/20) of those with RA exhibited abnormal dexamethasone suppression tests (p less than 0.001). No differences were noted in the diurnal variation of other hormones tested. Beck Depression Inventory scores were similar in both groups and no patient exhibited clinical evidence of depression. These data suggest alteration in the pituitary hypothalamic axis with respect to cortisol secretion in fibromyalgia syndrome, perhaps as a consequence of chronic pain.