To ensure appropriate fluid replacement, caregivers need to understand the effect of support surfaces on the rate of moisture loss from the body. A prospective study was conducted to 1) measure the rate of weight (fluid) loss on an air-fluidized therapy (AFT) surface; 2) determine the effect of bath temperature on weight loss; 3) compare weight loss and skin temperatures between foam and low-air loss (LAL) surfaces; 4) compare characteristics of individuals with high and low weight loss rates; and 5) compare weight loss rates to previously measured rates of support surface evaporative capacity. Eight healthy adult volunteers (three men, five women, average age 33 years, average body mass index 31.0 kg/m²) participated in eight 180-minute trials (one trial per day) on a foam, an LAL, and an AFT surface at five different bath temperatures (range: ~99.0 °F [hot] to ~88.0 °F [low]). Weight (± 10 g) was obtained before and after each trial and skin temperature (across the back) was recorded continuously. Using linear regression, weight loss rate on AFT was found to be strongly dependent upon bath temperatures: weight loss/day (g/m²-24 hours) = 53.9 x T (F) - 4030, where T is the mean skin temperature on the patient's back (also equal to bath temperature) in (F) Fahrenheit. Using this regression equation at mid-range (94 °F), fluid loss in an average woman (145 lb/64.5 inches/1.72 m² body surface area [BSA]) on AFT would be estimated to be 850 g/day higher than on foam. Compared with LAL, weight loss on AFT was estimated to be 700 g/day and 800 g/day higher for the typical woman and man, respectively (P < 0.05 at a mid-range bath temperature of 94 °F). Weight loss rates varied from 480 g/m²-24 hours to 3,470 g/m²-24 hours. Weight loss and mattress evaporative rates also suggest that moisture accumulation may occur on a foam but not on an LAL or AFT surface. However, fluid intake should be increased on AFT, particularly when bath temperature settings are high.