Introduction. Acute liver porphyrias are caused by mutations of genes encoding the heme biosynthesis enzymes. A lack of any of these enzymes leads to a decrease in the heme level, which in each case is manifested by identical clinical symptoms. Nervous tissue is particularly susceptible to a depletion of heme levels. Three classical symptoms of a porphyria attack include: abdominal pain (as a manifestation of autonomic neuropathy), peripheral neuropathy, and psychic disturbances. Apart from the main symptoms, porphyria attacks may be manifested as various neurological disturbances, such as: bulbar palsy, epileptic seizures, psychic disturbances and focal symptoms of CNS damage. The intensity of individual symptoms can vary. Although neurological symptoms usually disappear after the attack, they may cause permanent deficits. Through involving CNS, porphyria can mimic other neurological disorders. Damage to the nervous system can be confirmed by additional examinations. Thus far, disseminated focal changes during the attack, which disappear after the regression of clinical symptoms, have been described in magnetic resonance imaging (MRI) scans of the brain. Spinal cord abnormalities in MRI scans have not been described yet, despite the fact that damage to the anterior horn associated with porphyria has been confirmed in literature. While examining cerebrospinal fluid during the attack, an elevated protein level has been noticed, so far without the presence of oligoclonal bands. Aim. This paper aimed to demonstrate that porphyria should be considered in differential diagnosis of clinically isolated syndrome (CIS) and multiple sclerosis (MS). Materials and methods. This paper is based on the case of a symptomatic patient who carried a mutation in one of the genes for porphyria, and who developed a clinical manifestation of focal damage to CNS. A differential diagnosis of porphyria and CIS, with a subsequent conversion to a clinically defined MS, was carried out. The results of additional examinations performed routinely in both disorders, the clinical picture and data from medical literature have been used in the analysis of this case. Case study. The authors present the case of a patient with confirmed genetic porphyria, who was hospitalized for purposes of establishing a diagnosis due to weakened lower extremities. The patient manifested spastic lower extremity paresis, sensation disorders and urinary incontinence, which indicated spinal cord damages. An MRI scan of the thoracic spine revealed disseminated foci with contrast enhancement. A high level of proteins and oligoclonal bands were detected in the cerebrospinal fluid. On the basis of a new clinical picture (differing from the previous attack symptoms), prolonged symptoms, spinal cord lesions in MRI, and the presence of oligoclonal bands, CIS was diagnosed. Quick and complete recovery was obtained due to the administering of a steroid therapy. After several months, the patient reported a similar, passing episode, which, according to the authors, confirmed the diagnosis and an expected conversion to a clinically defined MS. Results and discussion. Taking into consideration the clinical manifestation of spinal cord involvement untypical of porphyria, the detection of spinal lesions in an MRI scan, the presence of oligoclonal bands in the cerebrospinal fluid and the positive effect of steroid therapy, the authors believe that this described patient, who was a symptomatic carrier of a mutation in a gene for porphyria, developed CIS. Conclusions. This reported case, supported by data from medical literature, shows that porphyria should be taken into consideration in any differential diagnosis of CIS and MS.