OBJECTIVE To assess whether recording of ambulatory blood pressure at night causes arousal from sleep and a change in the continuous blood pressure recorded simultaneously. METHODS Repeated measurement of blood pressure with two ambulatory blood pressure machines (Oxford Medical ABP and A&D TM2420) during continuous measurement of beat to beat blood pressure and continuous electroencephalography. METHODS Sleep research laboratory. METHODS Six normal subjects. METHODS The duration of electroencephalographic arousal and the beat to beat changes in blood pressure produced by the measurement of ambulatory blood pressure; the size of any changes that this arousal and change in blood pressure produced in the blood pressure recorded by the ambulatory machine. RESULTS Both ambulatory blood pressure machines caused arousal from sleep: the mean duration of arousal was 16 seconds (95% range 0-202) with the ABP and 8 seconds (0-73) with the TM2420. Both also caused a rise in beat to beat blood pressure. During non-rapid eye movement sleep, this rise led to the ABP machine overestimating the true systolic blood pressure during sleep by a mean of 10 (SD 14.8) mm Hg and the TM2420 by a mean of 6.3 (8.2) mm Hg. On average, diastolic pressure was not changed, but measurements in individual subjects changed by up to 23 mm Hg. These changes varied in size among subjects and stages of sleep and were seen after measurements that did not cause any electroencephalographic arousal. CONCLUSIONS Ambulatory blood pressure machines cause appreciable arousal from sleep and therefore alter the blood pressure that they are trying to record. This effect should be taken into account when recordings of blood pressure at night are interpreted in clinical work and epidemiological research.