To compare the effectiveness of multi-stage and one-stage alternating low-pressure air mattresses (ALPAM) and alternating pressure air mattress (APAM) overlays in preventing pressure ulcers among hospitalized patients, data were pooled (N = 617) from a study of patients allocated to multi-stage ALPAM (n = 252) or one-stage ALPAM (n = 264), and another study of patients allocated to APAM overlay (n = 101). Cumulative pressure ulcer incidence was 4.9% (n = 30) over 14 days. Fewer ulcers developed on multi-stage ALPAM compared with APAM overlay (OR = 0.33; 95% CI [0.11, 0.97]), but no difference was found between one-stage ALPAM and APAM overlay (OR = 0.40; 95% CI [0.14, 1.10]). Time to develop ulcers did not differ by mattress type. © 2013 Wiley Periodicals, Inc. Res Nurs Health 36: 439–452, 2013.