BACKGROUND: Blood pressure measurement are used by medical purposes. Most clinical staffs assume that its values are accurate and reliable irrespective of the measured position. In order to measure blood pressure exactly the cuffed upper arm and the heart should be kept at the same level. However, sometimes the blood pressure must be taken with the patient lying on his/her side. In this lateral position it is difficult to set the cuff at the heart's level. PURPOSE: This study aims at applying exact measurements of blood pressure to practical use. Blood pressure measurements taken from the four extremities (right arm, left arm, right leg and left leg) in the lateral position are compared to those taken in the right arm in the supine position. METHODS: Thirty-one female and Thirty-two male subjects were sampled among fit students and the data were collected from May 7th, 2001 through June 7th, 2001. To begin with, the blood pressure of right arm in the supine position was measured. Then the blood pressure was measured again after 3 minutes in the subject's right lateral position. RESULTS: The blood pressure in the left arm in the right lateral position was lower than the right arm's blood pressure in the supine position (systolic difference=15.57 mmHg, diastolic difference=10.86 mmHg). DISCUSSION: Hydrostatic effects are the most likely cause of the drop in blood pressure of left arm in right lateral position. When blood pressure is measured in the left arm in the right lateral position, the position and site should be noted as well as the pressure may be 10mmHg or more lower than the precise blood pressure.