Background: We investigated differences in the prevalence of anosognosia and neuropsychiatric symptoms (NPSs) characteristics according to disease severity in patients with early-onset Alzheimer disease (EOAD). Methods: We recruited 616 patients with EOAD. We subdivided participants into 2 groups based on the presence or absence of anosognosia and then again by Clinical Dementia Rating (CDR) scale. We compared the differences in the Neuropsychiatric Inventory (NPI) scores according to anosognosia and disease severity. Results: The percentage of patients with anosognosia in each CDR group steadily increased as the CDR rating increased (CDR 0.5 8.6% vs CDR 1 13.6% vs CDR 2 26.2%). The NPI total score was significantly higher in patients with anosognosia in the CDR 0.5 and 1 groups; by contrast, it had no association in the CDR 2 group. Frontal lobe functions were associated with anosognosia only in the CDR 0.5 and 1 groups. After stratification by CDR, in the CDR 0.5 group, the prevalence of agitation (P = .040) and appetite (P = .013) was significantly higher in patients with anosognosia. In the CDR 1 group, patients with anosognosia had a significantly higher prevalence of delusions (P = .032), hallucinations (P = .048), and sleep disturbances (P = .047). In the CDR 2 group, we found no statistical difference in the frequency of symptoms between patients with and without anosognosia. Conclusion: These results confirm that the prevalence of anosognosia as well as the individual NPS and cognitive functions associated with it differ according to EOAD severity.