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The aim of our study was to analyse the 24-h periodic pattern of blood pressure (BP) in diabetic patients with abnormal responses to cardiovascular reflexes, in order to evaluate the extent of the initial autonomic damage. We studied 44 patients with diabetes mellitus (14 insulin-dependent, 30 non-insulin-dependent; mean duration of disease 6.5±1.8 years) in good metabolic control (fasting glycaemia <140 mg/dl, postprandial glycaemia <180 mg/dl, fructosamine <285 mg/dl), divided into two subgroups, containing 21 normotensives (13 males and 8 females aged 28–72 years) and 23 hypertensives (13 males and 10 females aged 32–70 years) respectively. All patients showed abnormal responses to at least two out of four tests: deep breathing, lying to standing, Valsalva manœuvre and postural hypotension. Two sex-and age-matched control groups were recruited, comprising 20 normotensive and 20 hypertensive diabetic patients without dysautonomia, respectively. The reference group consisted of 248 normotensives (135 males and 113 females, aged 18–76 years) and 212 mild-moderate hypertensives (130 males and 82 females, aged 27–66 years). Each patient underwent ambulatory BP monitoring for at least 24 h, using an auscultatory automatic device. Data concerning biological rhythms were analysed by means of periodic functions. We limited the Fourier partial sums to the first three harmonics. In the diabetic normotensive groups, we observed that the absolute systolic and diastolic BP minima during the night occurred very rapidly and that the increase to the morning maximum was markedly slowed in patients with abnormal responses to cardiovascular tests in comparison with the controls (nocturnal BP decrease -5.8/-4.7 vs -3.8/-4.0 mm Hg/h; increase 4.7/3.6 vs 5.9/6.1 mm Hg/h). The same phenomenon was observed in both hypertensive groups but the differences were more marked (nocturnal BP decrase -7.7/-7.1 vs -4.3/-3.9 mm Hg/h; increase 3.2/2.1 vs 5.8/4.3 mm Hg/h). Diabetic patients without dysautonomia had similar patterns to those in the normotensive and hypertensive reference groups. Since alterations in circadian rhythm are preceded by an intermediate smoothed BP curve during the early morning, which is well defined by Fourier analysis and related “speeds”, this method makes it possible to quantify the extent of the initial autonomic damage.