A previous study reported that the midnight-to-morning urinary cortisol increment method could be used to reliably assess the insufficiency of the hypothalamic-pituitary-adrenal (HPA) axis. The principal aim of the present study is to verify whether the midnight-to-morning urinary cortisol increment is a reliable method for the assessment of the HPA axis in patients with various degrees of impaired kidney function. Fifty-six clinically stable patients with chronic kidney disease (CKD) and 14 healthy subjects were enrolled in the present study. Patients with CKD were divided on the basis of glomerular filtration rate (GFR) into the following arbitrary groups: mild (GFR: 60-89 ml/min/1.73 m2, no.=15), moderate (GFR: 30-59 ml/min/1.73 m2, no.=12) and severe kidney insufficiency (GFR: 15-29 ml/min/1.73 m2, no.=13), and hemodialysis patients. Plasma cortisol and ACTH levels were measured. The HPA axis was assessed by short Synacthen test and overnight dexamethasone suppression test. Double voided urine samples were collected at midnight and waking in the patients and the controls. Urinary free cortisol (UFC) and creatinine levels were measured and the UFC/creatinine ratio (Cort/Cr) was calculated. Then, the Cort/Cr increment was calculated as the morning Cort/Cr minus the midnight Cort/Cr. Baseline plasma cortisol levels were not significantly different between two groups. However, we found that CKD patients had significantly greater plasma ACTH levels than controls. The patients with CKD had also significantly lower morning UFC levels than controls and there was a progressive fall in morning UFC levels with decreasing GFR. The assessment of the HPA axis in patients with GFR lower than 29 ml/min was hampered by falsely abnormal responses to the midnight-to-morning urinary cortisol increment method. Plasma cortisol responded normally to exogenously administered ACTH, while plasma cortisol was suppressed by overnight dexamethasone administration in all patients with CKD. In conclusion, this method is not a reliable test for assessment of the HPA insufficiency in patients with GFR lower than 29 ml/min.