Introduction. Dissociative and conversion disorders are classified together according to ICD-10 as states that are not confirmed by the presence of somatic diseases, which they suggest. According to the DSM-IV, both disorders are classified separately. Conversion disorders are a group of psychiatric disorders whose symptoms mimic the presence of malfunction or loss of motor or sensory function, whereas the nature and dynamics of the observed symptoms is not fully explained by the results of objective assessments and consultations, nor is the direct effect of a psychoactive substance. Impaired mental integration of different functions which normally interact simultaneously in the perception of reality and inner experience of the individual is found in dissociative disorders. Aim. The article describes the case of 25-year old man, in whom after initial suspicion of myasthenia gravis and its exclusion, a diagnosis of conversion disorder was made on the basis of the clinical picture and treatment with an SSRI antidepressant and individual psychotherapy were recommended. No improvement in mental and neurological status after six month therapy resulted in an in-depth diagnostics in a clinical setting and diagnosis of brain stem tumor (aastrocytoma fibrillare). Conclusions. (a) Neuroimaging is a source of important clinical data and in many cases should constitute an inherent element of a psychiatric diagnosis. (b) Diagnosis of conversion (dissociative) disorders requires a precise differential diagnosis, excluding the somatic causes of observed neurological ailments. (c) A late diagnosis of neurological or somatic causes of symptoms which arouse a suspicion of conversion (dissociative) disorders may make a radical treatment impossible or may considerably aggravate the remote prognosis and quality of the patients' life.