We observed a young patient with slight mental retardation, suffering from drug-resistant tonic-clonic seizures, who presented a status epilepticus (SE), in two separate periods when valproic acid (VPA) was added to the phenobarbital (PB) already being used in the patient's therapy. The VPA-induced SE was characterized by normal plasma levels of antiepileptic drugs (AEDs), normal ammonemia and normal liver function. The case we studied represents the first report on a VPA-induced SE. Furthermore the case confirms that VPA, as well as causing encephalopathy secondary to hyperammonemia, may also provoke a primary involvement of the central nervous system (CNS), specially when used in young epileptic, mentally retarded subjects. In an attempt to explain the paradoxical effect of VPA we hypothesise that the SE could be due to an increase in excitatory activity producing a consequent epileptogenic effect in those subjects with a predisposition for toxic reaction to VPA therapy due to congenital anomalies of neural networks.