Background The aim of this study was to test the General Oral Health Assessment Index Questionnaire (GOHAI) items for differential item functioning (DIF) according to demographic characteristics (gender, age) and mental health status (schizophrenic disorders versus general population) using Rasch analysis. Methods This is a cross-sectional study using aggregated baseline data from a validation study of the GOHAI in the French general population (GP) and similar validation study in persons with schizophrenia (PWS). DIF was tested using the Partial Credit Model. DIF were estimated in different groups of patients. Results The cohort comprised 363 persons: 65% were female, 83% were 25-45 years old and 30% were PWS.Five of the 12 items exhibited DIF. DIF effects were observed with schizophrenia for 3 items, with age for 3 items and with gender for 1 item. The variable “age” gave a significant explanation of the latent variable: the latent variable decreased with age (-0.40±0.08-p<0.001 for each increase of the age of 10 years). This decrease represented an effect size of 0.27 which can be qualified of a small to medium effect. The status of the individuals (GP versus PWS) and the gender did not significantly explain differences in the values of the latent variable. Conclusion The GOHAI scores may not be comparable across sub-groups defined by health status, age and gender without accounting for DIF. In the future, other studies should explore this way with other Oral Health related Quality of Life assessment tools and populations with mental illness.