Microcellular polyurethane flexible foams having densities no greater than 0.3 g/cc which are suitable for use as lightweight shoe sole components are produced with carbon dioxide in an amount such that the polyurethane-forming mixture has a free rise density of from about 0.03 to about 0.3 g/cc. At least a portion of that carbon dioxide is dissolved as a gas into one or both of the reaction components. The amount of dissolved carbon dioxide must be such that the froth density of the isocyanate and/or isocyanate-reactive component(s) in which the carbon dioxide is dissolved will be from about 0.1 to about 0.8 g/cc. Additional carbon dioxide may be formed by the reaction of water and isocyanate during the polyurethane-forming reaction but the total amount of CO.sub.2 present should be controlled to ensure that the polyurethane-forming mixture has a free rise density of from about 0.03 to 0.3 g/cc. Use of a preferred isocyanate-reactive component in which a specified ratio of diol to triol is satisfied makes it possible to use more water than had been expected. Use of a preferred prepolymer makes it possible to produce microcellular polyurethanes having good physical properties solely with a diol. The product microcellular foams possess a uniform cell structure and enhanced physical properties as compared to all water-blown foams of the same basic formulation and density. The hardness of the foams is more suitable for shoe sole, particularly midsole applications, than that of the water-blown foams, despite the lower urea hard segment content of the CO.sub.2 blown foams.