Acute psychotic states develop in 2—3% of patients with multiple sclerosis (MS). At the stage of differential diagnosis, between the organically caused and endogenous nature of psychotic disorders, it is necessary to take into account their connection with the period of exacerbation of the disease, the presence of mental disorders during the period of remission and before the development of MS, the possibility of a reaction to pulse therapy with corticosteroids and antipsychotics. The article presents a case with an emphasis on the features of diagnosis and drug treatment of an acute psychotic state, which developed several years after the diagnosis of MS. Considering the rarity of organic delusional disorder in connection with MS and the limited number of scientific studies, as well as the difficulty of selecting antipsychotic drugs, the case helps to highlight the features of diagnosing and selecting pathogenetic therapy for an acute psychotic state.