Six hundred patients at risk for pressure sores were randomized in either a control group or one of two experimental groups placed on alternating-pressure air-mattresses and water-mattresses. The groups remained comparable throughout the 10-day study period. Twenty-one patients from the control group developed decubitus ulcers, compared with 7 in each of the other groups. Patient and ward personnel opinions on the acceptability of the three types of mattresses were registered.