Paraneoplastic syndromes, including paraneoplastic dermatoses, represent a pressing problem for dermatovenereologists, infectious disease specialists and therapists due to the necessity of having broad knowledge in related disciplines. Our article examines a clinical case of the onset of paraneorplastic polydermatomyositis against the background of a new coronavirus infection in a patient with exacerbation of paranoid schizophrenia. When paraneoplastic syndrome manifested itself in this clinical situation, a differential diagnostic search was carried out at the junction of several nosological units, such as the locomotor primary latent form of brucellosis, trichinosis, polydermatomyositis, and steroid myopathy. Taking into account the epidemiological history and the fact of the patient’s residing in a rural area, it was first necessary to exclude the infectious nature of these manifestations, as well as autoimmune pathology involving muscles and skin. A wide range of laboratory and instrumental examination methods were carried out, including false-positive IgM results for the causative agent of brucellosis, which can be explained within the framework of paraneoplastic syndrome and the tumor’s synthesis of biologically active substances. The results formed the basis for the entire clinical picture of the disease. Identification of the oncological process, which became possible with the appearance of pathognomonic clinical symptoms, made it possible to establish the histological picture of the disease and to launch treatment for this patient. In addition, the final clinical diagnosis was complicated by the presence of an underlying disease that required constant use of antipsychotics and the lack of control of the ongoing basic pharmacotherapy in the form of monitoring prolactin levels and assessing the general condition of the body systems.