Objective: Study the dimensional changes on gypsum casts poured with 2 addition silicones and a polyether, after the impressions were steam autoclave sterilized and stored. Material and Methods: 2 commercial brands of addition silicones and a polyether were used in an automatic mixing machine, Pentamix 2 (3M ESPETM): Aquasil Ultra DECATM Monophase (Dentsply); Normosil Adición Putty Fast (Normon©); ImpregumTM PentaTM Soft Polyether Impression Material (3M ESPETM). 10 samples of each impression material were obtained with a metallic matrix from ADA Specification n° 19 (1977) and ISO 4823:1989. The matrix was filled up with each impression material. They were didvided into 2 groups: A subjected to chemical disinfection with sodium hypochlorite 5.25% for 10 minutes; and B were submitted to autoclaving at 134°C for 25 minutes, in addition to the disinfection. The gypsum casts were made from each sample: after the procedure, 24 hours, at two weeks and at 6 months, and measured with a digital caliper. We measured three master lines in each cast, a), b) and c). Results: The Aquasil Ultra DECATM Monophase (Dentsply) casts had the most similar dimensions to the matrix. The Normosil Adición Putty Fast (Normon©) casts showed the most evident differences. Conclusion: The studied impression materials suffered dimensional changes. However, according to the ADA Specification n° 19 (1977) these changes aren't significant for they don't exceed 0,5%. There for we have come to the conclusion that these materials can be submitted to steam autoclave sterilization and stored for a long time. Keywords: Dimensional changes, Disinfection/sterilization, Gypsum products and Impression materials