This study investigated whether nondipping (defined as a day-night change in blood pressure (BP) <or=0%) could be assumed as a diagnostic index for autonomic neuropathy, and assessed its accuracy in discriminating between type I diabetic patients with and without autonomic neuropathy. In 87 type I diabetic patients with normal renal function (age 36+/-11, duration 17+/-9 years, serum creatinine 67.2+/-15.9 micromol/l), four cardiovascular tests and 24-h BP monitoring were performed, and the percentage day-night change (Delta) in systolic (SBP) and diastolic BP (DBP) was calculated. Sixteen patients had DeltaSBP and/or DeltaDBP <or=0%. In a multiple logistic regression with adjustment for sex, age, and body mass index, the odds ratio for having autonomic neuropathy was seven times higher in patients with DeltaSBP <or=0% as opposed to those without (odds ratio 6.97, CI 1.4-34.9, P=0.018). Using Receiver Operating Characteristic (ROC) analysis, DeltaBP showed an acceptable accuracy in discriminating between patients with and without autonomic neuropathy (area under the ROC curve 0.69+/-0.06 and 0.72+/-0.05 for DeltaSBP and DeltaDBP, respectively). Adequate cutoff values were 0% for DeltaSBP (sensitivity, 26%; specificity, 95%; positive predictive value, 87%) and 5% for DeltaDBP (sensitivity, 26%; specificity, 92%; positive predictive value, 81%). In type I diabetic patients with normal renal function, a value of DeltaSBP <or=0% identifies the presence of autonomic neuropathy with a very high chance. Nondipping at the cutoff proposed could be considered an adjunctive marker of autonomic neuropathy provided with a high specificity and low sensitivity.