Introduction: The success of oral rehabilitation depends on the accuracy and reproducibility of the impressions. Objective: The purpose is to study the dimensional changes of a polyether and addition silicone subjected to disinfection and sterilization after a long storage period. Materials and Methods: 90 samples were obtained from polyether Impregum Penta (3M ESPETM) and 90 of the addition silicone Imprint Penta Putty (3M ESPETM) according to ISO 4823:2000 and stored in the IPQ (Portuguese Institute for Quality) for six months. The measurements were performed by laser interferometry, according to the Michelson technique. The dimensional stability was calculated according to the formula specified in ISO 4823:2000. Results: A statistical analysis via a t-distribution for paired samples with a confidence interval of 95% was performed. The statistically significant expansion of Impregum Penta (3M ESPETM) was 0,77±0,17% in the control group, 0,42±0,19% in the hypochlorite group and 0,52±0,28% in the autoclave group. In the Imprint 4 Penta Putty (3M ESPETM), the control group had an expansion of 0,42±0,12%, the hypochlorite group 0,36±0,09% and the autoclave group 0,59±0,13%. Conclusion: The tested impression materials exhibit dimensional changes when subjected to disinfection and sterilization. These are not clinically significant because they are within the maximum extent permitted by the ISO 4823:2000. Clinical implications: The dimensional stability of impression materials subjected to disinfection, sterilization and long storage periods are critical to the success of the final prosthetic restoration. Schlagwörter: Polyether, addition silicone, disinfection, sterilization, laser interferometry, dimensional stability