Fourteen diabetic patients (13 males, 1 female, 7 IDDM and 7 NIDDM) were tested during wakefulness with a battery of tests examining parasympathetic and sympathetic control of the cardiovascular system. Subsequently sleep recordings including EEG, EOG, submental, left and right anterior tibialis EMGs, ECG, nasal airflow, thoracic and abdominal respirograms, nocturnal penile tumescence, were performed in each subject. The assessment of cardiovascular functions during sleep was based on the following parameters: Rbm, R-wake, apnea index, adequate penile tumescence during phase REM. Parasympathetic and sympathetic control of cardiovascular system were both impaired during wakefulness in only one patient, who also showed a low Rbm index indicative of ascertained autonomic neuropathy. Indices Rbm sufficiently low to be considered an evidence of probable autonomic neuropathy were found in 5 patients (3 IDDM and 2 NIDDM); all but one with normal cardiovascular tests during wakefulness. Five patients showed gross deficiency upon nocturnal penile tumescence monitoring. In comparison with a control group the patients showed a significantly lower overall Rbm index (p less than 0.001). IDDM patients showed an increased plasma GH response to insulin-induced hypoglycemia compared to NIDDM and normal subjects. Increased GH secretion was furthermore confirmed by GH values obtained in blood samples drawn during the first REM stage of the night in IDDM patients. The evaluation of the variables taken into consideration during sleep appears to be crucial for the assessment and prevention of autonomic neuropathies and neuroendocrine dysregulation in diabetic patients.