In order to assess the characteristics of day-night blood pressure (BP) variation in normotensive and hypertensive non-insulin-dependent diabetic (NIDDM) patients with asymptomatic autonomic neuropathy, 54 NIDDM patients and 13 healthy control subjects were studied by casual BP measurements and 24-h ambulatory blood pressure monitoring. Signs but not symptoms of autonomic neuropathy were documented by results of standard cardiovascular function tests in each patient. Daytime (06:00-22:00) and nighttime (22:00-06:00) BP values were separately analyzed and delta day-night BP values and diurnal index were determined. Patients were classified as being normotensive or having hypertension according to the casual BP values and medical history. In normotensive NIDDM patients (n = 30), nighttime systolic BP was significantly higher, whereas delta day-night systolic and delta day night diastolic BP values as well as diurnal index were considerably lower than those in control subjects (n = 13). In hypertensive NIDDM patients (n = 24), similar alterations were found at higher BP levels. No significant difference was found in BP values if normoalbuminuric and microalbuminuric NIDDM patients were compared. 'Non-dipper' phenomenon could be found in normotensive and hypertensive NIDDM patients with asymptomatic autonomic neuropathy, suggesting that relative sympathetic overdrive due to incipient and predominantly parasympathetic impairment of cardiovascular innervation might play a role in early alterations of circadian BP variation.