[reaction: see text] The solution and solid-phase synthesis of imidazolones is reported. The key step for the preparation of these compounds is the N-H insertion reaction of primary ureas into highly reactive rhodium carbenoid intermediates. Typically, a soluble or support-bound alpha-diazo-beta-ketoester is treated with a rhodium carboxylate catalyst in the presence of a primary urea to give the corresponding N-H insertion product. Subsequent acid-catalyzed cyclodehydration of these insertion products affords the desired imidazolone products.