When selecting a stock tray for an impression in a fully dentate patient the dental practitioner is often confronted with the problem of finding an impression tray that fits exactly. This problem is stated in many textbooks on prosthodontics but scientific data are lacking. Objective: Thus it was the objective of this study to analyze the geometry of the dental arches of fully dentate subjects and compare it to the geometry of different stock trays. Material and Method: The geometry of studycasts [1479 upper jaw (UJ), 1544 lower jaw (LJ)] of randomly selected fully dentate adult individuals was assessed and electronically compared to each tray of a whole set of Aesculap (AS), Algilock (AL), Ehrike, Inox Anatomic (IA), and Martin Elite stock trays. For each set of trays the amount of casts was calculated that fitted at least one tray. The data were subjected to a chi-square test and to a cluster analysis. Results: Results ranged from 35.6% (AL) to 92.4% (AS) for the UJ and from 13.6%  (IA) to 52.0% (AS) for the LJ (sign., p<0.01). Results for the UJ were significantly (p<0.01) better than for the LJ. Cluster analysis revealed that there was a discrepancy in the variation of the casts and the shape of the trays. Conclusion: Within the limits of this study it can concluded that the accuracy of fit of commonly used stock trays – especially for the LJ – lies beyond clinically acceptable ranges and therefore should be improved. Therefore a prototype set of 4 trays per jaw was designed based on the data and fabricated (3M ESPE). The accuracy of fit of these prototypes was assed in 125 other subjects. Only in 2% (LJ) and 8% (UJ) none of the trays fitted. This indicates that it is rather possible to improve the quality of fit of stock trays.

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