Interface pressure measurement is needed to assess beds designed to prevent pressure sores, so it is therefore important to establish the accuracy of interface pressure measuring systems. In this study, the Talley SA500 pressure evaluator (with 28 mm and 100 mm sensor pads), the DIPE (with 100 mm sensor pad), and a water-filled bladder system (with 0.1 ml and 0.3 ml water) were assessed. Measurement errors were evaluated using a loading system with pressures up to 7.4 kPa (55 mm Hg) in steps of 0.9 kPa (6.9 mm Hg). All systems tested over-measured interface pressure, the error being approximately linearly proportional to the loading pressure. The repeatability for a given system was approximately constant. The mean error (+/- SD) (%) and repeatability (kPa) for the systems were: 28 mm Talley 12 +/- 1%, +/- 0.07 kPa; 100 mm Talley 15 +/- 1%, +/- 0.07 kPa; DIPE 27 +/- 3%, +/- 0.12 kPa; 0.1 ml water bladder 17 +/- 1%, +/- 0.13 kPa; 0.3 ml water bladder 26 +/- 3%, +/- 0.07 kPa. Different interfaces affected accuracy markedly, and repeatability was affected when an inhomogeneous interface was used. The study shows that the errors associated with interface pressure measurement systems can be substantial, and can vary from one system to another.