Introduction: synthetic resins are currently used with academic proposals to prepare anatomic organs by using injection techniques through blood vessels, conducts and hollow organs. Finding resin’s right conditions and physical properties according to what process needs, has been difficult. Some evidences indicate that styrene plays an important role in changes in resin physical properties. Objective: To test the effect of styrene in different quantities in a resin flexible mixture over tensile strength. Materials and methods: Three resin mixtures were prepared with 10, 20 and 30% styrene (A, B and C respectively) and four standard models were built by using each mixture. Those models were placed in an Assay Universal Machine until they broke, testing their traction resistance. Results: Models in A mixture broke at 148.75kg.-f median charges, B mixture broke at 161.25 and C mixture at 296.25, so there is a significant difference between C and the other two groups. Conclusions: Our results suggest that styrene addition 30% in the flexible polyester resin increases the tensile strength. It is necessary probe this performance in different resins and at different styrene quantities. Salud UIS 2008, 40: 191 - 196