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The effectiveness of a passive cooling ice vest in reducing heat strain was evaluated. The SteeleVest is a canvas vest with pockets which hold frozen gel packs against the torso (total weight 5.1 kg). Eight test volunteers attempted 10, 4-hour heat exposures (five environments, with and without the vest) in hot-humid and hot-dry environments having Wet Bulb Globe Temperatures (WBGT) of 36-39 C. Dry bulb temperatures ranged from 38-49 C, with 25-80% relative humidity. Subjects wore the Navy utility uniform and exercised at 272 W. This corresponded to a work rate equivalent to the Navy PHEL Curve V. In all environments, the SteeleVest significantly reduced thermal strain, as evidenced by reduced rectal and skin temperatures, heart rate and sweat rate. Use of the SteeleVest approximately doubled tolerance times compared with tests without the vest. The gel packs lasted approximately 2 hours before they required replacement. When the hot-humid and hot-dry environments having equivalent WGBTs were compared, thermal strain was higher in the more humid environments. In addition to its effectiveness in reducing heat strain, the SteeleVest is relatively lightweight, has a low profile, requires little maintenance and is not susceptible to mechanical problems. These characteristics make it potentially feasible for shipboard use.