Abstract: Disclosed are a pressure-sensitive adhesive composition comprising a reactive mixture of a relatively low molecular weight acrylic prepolymer containing highly reactive hydroxyl groups in the molecule thereof which is obtained by copolymerizing two alkyl acrylate monomers satisfying a specific relationship and, as a chain extender, a polyfunctional isocyanate mainly exhibiting bifunctionality; a pressure-sensitive adhesive composition comprising the above reactive mixture and a relatively high molecular weight polymer component; and pressure-sensitive adhesive sheets having a pressure-sensitive adhesive layer comprising those pressure-sensitive adhesive compositions. Because the molecular chain of the acrylic prepolymer is extended efficiently, the amount of an organic solvent required for viscosity adjustment can be decreased.