Purpose: The aim of this study was to investigate the effect of case severity on clinical outcomes when fabricating new complete dentures. Materials and Methods: Participants were separated into severe and moderate groups using the index of case difficulty for edentulous patients developed by the Japan Prosthodontic Society. Before and after treatment, self-assessed masticatory ability and oral health-related quality of life (OHRQoL) were examined, and the authors compared them according to case severity using the Mann-Whitney U test. To compare findings before and after treatment, the authors used the Wilcoxon signed rank test. Results: In the severe group, both scores were significantly improved after treatment (P .01). However, in the moderate group, there was no significant difference in self-assessed masticatory ability as measured by the food acceptance score before and after treatment (P = .11). Before treatment, OHRQoL as measured by the Oral Health Impact Profile score was significantly higher in the severe group than in the moderate group (P .01). However, after treatment, there was no significant difference between the two groups (P = .92). Conclusions: The authors concluded that case severity makes a difference in the edentulous patient's OHRQoL and self-assessed masticatory ability during complete denture treatment. Evaluating case severity with the index before treatment is a useful tool for patients and clinicians to predict clinical outcomes.