The invention relates to a process for preparing high temperature-resistant dense or cellular polyurethane elastomers by reacting the following, preferably using low-pressure processing techniques:(a) organic polyisocyanates, preferably in the form of isocyanate group-containing prepolymers and/or modified urethane group-containing organic polyisocyanates, with(b) relatively high molecular weight compounds having at least 2 reactive hydrogen atoms,(c) urea derivatives and, optionally,(d) chain extenders and/or cross-linking agents in the presence of(e) catalysts and, optionally,(f) blowing agents(g) auxiliaries and/or additiveswherein amino and/or hydroxyl group-substituted ureas and/or polyureas having molecular weights from 200 to 4000 are also used as the urea derivatives. Said ureas and/or polyureas are prepared by reacting organic diisocyanates with primary aliphatic and/or cycloaliphatic diamines and/or alkanolamines in molar ratios of from 1:1.1 to 1:2, preferably in the presence of polyester or polyether polyols as solvents or diluting agents.