###### Abstract  
The aim of the present in vitro study was to investigate the effect of using an autoclave with three different programs and the traditional water bath method (short cycle) on the changes in the vertical occlusal dimension of the complete acrylic dentures. Forty complete acrylic dentures were divided into four groups: G1, dentures were polymerized by water bath method (short cycle for 90 min at 74 \({}^{\circ}\)C and then 30 min at 100 \({}^{\circ}\)C); G2, dentures were polymerized with an autoclave (134 \({}^{\circ}\)C/2.1 bar/10 min); G3, dentures were polymerized with an autoclave (121 \({}^{\circ}\)C/1.1 bar/20 min); and G4, dentures were polymerized with an autoclave (121 \({}^{\circ}\)C/1.1 bar/5 min). The vertical occlusal dimension of the dentures was measured before and after the polymerization process. The difference between the initial and final measurements expressed the change in the vertical occlusal dimension. Data were statistically analyzed by \(t\)-test and one-way ANOVA test. Statistically significant lower vertical dimension changes were detected in G1 (0.703 \(\pm\) 0.078 mm) compared to the other groups (\(p<\) 0.05), whilst statistically greater changes were detected in G4 (1.263 \(\pm\) 0.063 mm) compared to the other groups. No statistically significant differences were found between G2 (0.844 \(\pm\) 0.061 mm) and G3 (0.88 \(\pm\) 0.059 mm) (\(p>\) 0.05). Within the limitations of the present in vitro study, it can be concluded that all polymerization methods increase the vertical occlusal dimension of complete acrylic dentures. The increase in the vertical occlusal dimension in Groups 1, 2 and 3 was less than 1 mm which is clinically acceptable and can be overcome by selective abrasion. Therefore, the traditional curing technique using a water bath and the autoclave methods using two settings (134 \({}^{\circ}\)C/2.1 bar/10 min or 121 \({}^{\circ}\)C/1.1 bar/20 min) could be used for the complete acrylic denture with acceptable augmentation in the vertical occlusal dimension. In contrast, the method used in G4 increases the vertical occlusal dimension by more than 1 mm, which is not clinically acceptable. vertical occlusal dimension; autoclave; polymerization; complete acrylic dentures 202320231360313 in dimensions [6; 7]. Therefore, the vertical occlusal dimension must be adjusted when delivering the dentures, and this procedure requires a long time to accomplish, resulting in a loss of the anatomical shape of the artificial teeth [8; 9]. Polymethyl methacrylate treatment and polymerization methods were developed over the years to improve its physical and mechanical properties. Several methods have been used, such as heat, light, chemicals, microwave energy and curing by pressure and heat device (autoclave) [10]. The water bath technique was used as a traditional method for curing thermosetting acryl due to its simplicity and low cost. However, this technique suffered from a major drawback which is the long time required for the curing process [11]. A pressurized steam device was used to harden acrylic bases for removable dentures for the first time by [PERSON] in 1976 [12]. This method of curing with a pressure device (autoclave) depends on raising the pressure and water temperature above 100 \({}^{\circ}\)C in a vacuum chamber. This method showed better results with its fast and slow cycles in terms of mechanical properties [13]. The physical and mechanical properties of the acryl used in the fabrication of dentures were studied by [PERSON] et al. [14]. They showed no difference in hardness between the prepared materials using the traditional method compared to an automatic pressure cooker. In contrast, they reported longer time for the traditional method than that of the automatic pressure cooker. Several studies have also been conducted on the autoclave curing technique, and [PERSON] et al. [15] reported that curing by using an autoclave ameliorates the mechanical properties of the curing acryl denture bases compared to the water bath curing technique. [PERSON] et al.