The characteristics of the curve of Spee in the mandibular arch have been extensively investigated. However, few studies have examined the characteristics of the curve of Spee in the maxillary arch. The purpose of this study was to examine the differences in the curve of Spee between the maxillary and mandibular arches. The effects of gender on the curve of Spee were also investigated. Fifty Japanese adults (25 men and 25 women) with permanent healthy dentitions participated. Standardized digital pictures of the right side of maxillary and mandibular dental casts were made with a digital camera. The cusp tips of the molars, premolars, and canines of the maxilla and mandible were identified. The radius and the depth of the curve of Spee were measured on the dental casts by means of computer software (Occlcircle). The Mann-Whitney test and the Wilcoxon signed rank test were used to test the statistical significance (α=.05). The curve of Spee showed a mean radius of approximately 106.4 mm in the maxillary arch and 83.4 mm in the mandibular arch. Radii of the curves of Spee in the maxillary arch were significantly larger than those in the mandibular arch (P<.0001) and had a depth of approximately 1.6 mm in the maxillary arch and 1.9 mm in the mandibular arch. The depth of the curve of Spee in the mandibular arch was significantly deeper than that in the maxillary arch (P=.0326). The curve of Spee was not influenced by the gender of the subjects investigated. The shape of the curve of Spee in the maxillary arch was significantly flatter than that in the mandibular arch.