Objectives: Flexure of the mandible when opening and closing of the mouth has been well documented. This preliminary study compared differences of making impressions with open and closed mouth impression methods by means of type of tray used. Methods: A full arch dual tray (DT) and a stock tray (ST) were used. A regular consistency vinyl polysiloxane impression material (Examix, GC) was used according to manufacturer's instructions. Impression material was auto-mixed and injected in the tray. Five mandibular impressions for each tray were made of an individual for a total of 10 impressions. The average mouth opening when using the stock tray was 20 mm between the tray and the incisal edge of tooth #8. Impressions were poured in a type IV stone (Resin Rock, WhipMix) according to manufacturer's specifications. Cast were trimmed and numbered in random order. Distinct reference anatomical points on the mesial fossae of the contralateral first mandibular molars (M) and distal fossae of the 2nd premolars (P) were selected. The linear intermolar was measured 5 times for each cast by one of the operators and the premolars by another operator with a electronic digital caliper (Tide Machine Tool Supply Co., Ltd ). Differences between open and close mouth impression were calculated by subtracting the intermolar/premolar distance. A power analysis (=0.95) showed the ability to detect a difference of 0.5 mm and was based on the pooled error standard deviation of 0.1863. Means and standard errors were calculated. Results: The mean intermolar/bicspid distance were M, ST: 30.42mm: M, DT: 30.58; B, ST: 27.34; B, DT: 27.44. A repeated-measures ANOVA showed no significant differences between the reference points for the two types of impression trays used. Conclusion: These results suggest that there are no significant differences between casts obtained from impressions made with open and closed mouth.