Metallic preformed steel crowns are the best treatment choice to restaurate extense cavities in temporary molars. To achieve optimal results it is necessary to know the mesiodistal width of the temporary teeth involved. We currently use other population’s data regarding mesiodistal width, and since we believe there could be significant variations in between populations we decided to study mesiodistal width in our patients. The purpose of this investigation was to determine mesiodistal widths of healthy temporary molars in Chilean children. Sexual dimorphism, interarch symmetry and with the contralateral tooth, and comparison with the standard preformed steel crown were analyzed. This is a descriptive odonthometric study. 34 children (17 male, 17 female) assisting to the Pediatric Clinic were selected. Mesiodistal widths were assessed with an orthodontic caliper on a plaster model obtained from each patient. Simultaneously, preformed steel crown mesiodistal widths were measured. Results and conclusions: There were no statistically significant differences between right and left teeth’s mesiodistal width, nor between genders. The mean mesiodistal width of temporary molars corresponds with only one size of preformed steel crown, except on 5.5 and 8.5. In these teeth the width corresponds with two sizes of the preformed steel crown.