The objective of the study was to determine the effect of the use of cuffs with different bladder sizes on the outcome of blood pressure (BP) measurements. Two sizes of bladders previously in common use in the study location (13 x 23 cm and 16 x 23 cm) were compared with the current size (13 x 36 cm). A fully randomised, experimental study was carried out on a study population comprising 130 subjects (61 men, 69 women, aged between 22 and 70, mean 49) who were not undergoing special treatment for cardiovascular disease or hypertension at that time. The mean arm circumference was 32.9 cm (range 25-40 cm). BP measurements were carried out by one person, using a Hawksley random zero sphygmomanometer under standardised conditions. With the smallest bladder (13 x 23 cm) the highest systolic and diastolic BP was measured (mean SBP 127.2 mean DBP 77.0 mm Hg), followed by the bladder of 13 x 36 cm (125.1 resp. 75.4 mm Hg). The lowest BP was measured with the bladder of 16 x 23 cm (123.7 resp. 74.4 mm Hg). Differences between bladders were significant for all arm circumferences. Over the entire range of arm circumferences in the present study there is a small systematic difference in the BP measurements taken by the various bladder sizes. For individual readings the difference is less marked, in comparison to other factors that can affect BP measurements. However, in longitudinal studies a systematic error in BP can occur when measurements are made with different bladders during the study.