After general considerations on diseases with alterations of the porphyrin metabolism a case of acute form of porphyria is reported. The patient, 29 years old, female, with pains in the arms and legs and abdominal colics, suddenly, presented motor paralysis of arms, legs and some of the cranial nerves. Confirming the diagnosis of porphyria, the Watson and Schwartz test demonstrated presence of porphobilinogen in the urine. After being interned in the hospital for a whole year, during which the disease developed in surges, the patient died. The anatomo-pathological study of the nervous system showed degenerative alterations of motor cells of the anterior horns and mesencephalic motor nuclei.