Objectives: It is well known that residual methyl methacrylate (MMA) reduces in autopolymerizing acrylic resins after additional, post-polymerization heat treatments. Residual MMA levels and in vitro cytotoxicity of hydroflask cured autopolymerizing acrylic resins subjected to post-polymerization heat treatments has not been well-documented. This study investigated and compared the residual MMA concentration ([MMA]r) and in vitro cytotoxicity of bench or hydroflask cured autopolymerizing resin with additional post-polymerization heat treatments, such as water storage at 60ºC or microwaving. Methods: A total of 288 acrylic resin specimens were prepared by bench (control) or hydroflask curing. Specimens were then subjected to post-polymerization heat treatments: a) water storage at 60ºC for 30 min, b) microwave polymerization at 500 W for 3 min, c) the combination of heat treatments of a&b. Specimens were then extracted in a complete cell culture medium for 1, 2 , 5 and 7 days. [MMA]r in extracts and cell proliferation of L-929 fibroblasts were measured using high-performance liquid chromatography and XTT assay, respectively. Data were statistically analyzed with Dunn's multiple comparison and Pearson correlation tests (p<0.05). Results: End of all extraction periods [MMA]r was significantly higher in bench cured specimens with no post-polymerization heat treatments (p<0.05) whereas cell proliferation was significantly higher in hydroflask cured specimens subjected to water storage at 60ºC for 30 min. End of 7 days of extraction, lowest [MMA]r were detected in hydroflask cured specimens subjected to both water storage at 60ºC and microwaving. The correlation between [MMA]r and cell proliferation was found negative at the end of all periods where it was statistically significant at the end of day 5. Conclusion: Hydroflask curing of autopolymerizing resin and also post-polymerization by water storage at 60ºC is recommended to reduce the leaching of residual MMA and in vitro cytotoxicity for clinical use.