The objective of this study was to evaluate the use of a polybutene additive to improve the flexure strength of two heat cured acrylic systems. A 920 g/mole polybutene, H100 (Amoco Chemical Co.) was chosen based upon previous work. The denture acrylics were Lucitone 199 and Lucitone Hy-Pro (Caulk Dentsply, Milford, DE). The polybutene was dissolved at one weight % in the monomer before mixing using the manufacturer's directions. The material was compression molded into rectangular specimens 10 mm wide/spl times/3 mm thick/spl times/75 mm long and cured for 9 hours in a hot water bath. Ten specimens were prepared for each formulation and stored for 48 hours in de-ionized water at 37/spl deg/C before mechanical testing. A three point flexure test was run using a span of 50 mm at a rate of 1 mm/minute. The results were compared using the Student-Newman-Kuels test (p/spl les/.05). Incorporation of the polybutene increased the flexure strength by 15% and modulus by approximately 40% for the Hy-Pro denture acrylic. The additive did not have a significant effect on the rubber modified L199 denture acrylic.