The blood pressure in both arms of 103 unselected hemiplegic patients was measured using a random-zero sphygmomanometer. Although for the whole sample the mean blood pressure in the paretic and unaffected arm was similar, a significant difference was found when the patients were subdivided according to the tone of the arm. The blood pressure was higher in paretic arms of patients with a spastic stroke and lower in the affected arm if the tone was flaccid. No other characteristics were associated with significant blood pressure differences, so that the findings appear to be directly related to changes in muscle tone. After a stroke the blood pressure should always be measured in the unaffected arm because changes in tone make measurements unreliable.