The purpose of this study was to assess postural stability in the medial-lateral (ML) direction when carrying unilateral and bilateral loads during stair negotiation. Twenty-four healthy young adults were instructed to ascend and descend a three step staircase under three load conditions: no load, 20% body mass (BM) bilateral load, and 20% BM unilateral load. A modified time-to-contact (TTC) method was proposed to evaluate postural stability during stair negotiation. Carrying unilateral loads required more rapid postural adjustments as evidenced by lower minimum ML TTC and ML TTC percentage as compared bilateral loads and no load during stair descent. In addition, lower ML TTC and TTC percentage were found for loaded limb stance for stair descent. Taken together, unilateral loads and the loaded leg during stair descent are of concern when considering postural stability during load carriage. These results illustrate differing postural control challenges for stair ascent and descent during load carriage.