Background: The extent to which MS patients with cognitive dysfunction can accurately self-report outcomes has been a crucial issue. The aim of this study was to quantify and compare the relevance of the quality of life (QoL) assessment between two populations with a high occurrence of cognitive dysfunction, specifically in individuals with multiple sclerosis (MS) and in individuals suffering from schizophrenia (SCZ). Methods: Design: A cross-sectional study was performed using the following inclusion criteria: MS and SCZ patients were diagnosed according to the McDonald criteria and DSM-IV criteria, respectively. Data on sociodemographic (age, gender, education level) and clinical (disease severity, disease duration) factors, QoL (disease-specific questionnaires, MusiQoL and SQoL) and cognitive performance (executive, memory, and attention functions) were collected. Non-impaired and impaired populations were defined according to the French norms. Psychometric properties were compared to those reported in reference populations, which were assessed in the respective validation studies. Suitability indices were provided used to quantitatively compare how the structures in the different populations matched with the initial structure of the questionnaires (reference populations). Results: One hundred and twenty-four MS patients and 113 SCZ patients were enrolled. Factor analysis was performed on the impaired populations and revealed that the questionnaire structure adequately matched the initial structure of the disease-specific QoL questionnaires. All of the suitability indices of construct and external validity in the non-impaired populations ranged from 70 to 100%. Conclusions: Our study suggested that cognitive dysfunction did not compromise the reliability or validity of the self-reported QoL questionnaires among subjects with cognitive dysfunction, such as MS and SCZ. Thus, this report may clarify the relevance of using self-reported QoL assessments in clinical practice.