1. The aim of the study was to assess the effect of ambulatory monitoring of blood pressure on sleep and on blood pressure in middle-aged patients. 2. Nine consecutive patients (seven men, two women; mean age 57 years) complaining of snoring and various degrees of excessive daytime somnolence were studied. Five patients were normotensive and four were being treated for hypertension. During one night standard laboratory polysomnography was performed with monitoring of blood pressure by a silent ambulatory monitor and continuous infrared blood pressure by photoplethysmography. 3. Ambulatory blood pressure significantly disturbs sleep architecture, causing EEG arousals in 64% of measurements, and induces a significant rise in blood pressure during systolic pressure measurement by the ambulatory monitor (rise in systolic pressure, 13.7 +/- 15.9 mmHg, P < 0.001; rise in diastolic pressure, 3.7 +/- 8.2 mmHg, P < 0.01). At the time of diastolic measurement, blood pressure had returned to the preinflation value. The rise in systolic blood pressure was higher when an arousal was associated with cuff inflation (P < 0.001). This rise in blood pressure is probably the consequence of sympathetic nervous system activation. 4. We conclude that ambulatory blood pressure recordings of systolic blood pressure during sleep should be interpreted with caution as systolic blood pressure may be significantly increased in patients suspected of suffering from sleep-disordered breathing.