We report a unique patient who had temporary isoniazid-induced myoclonic seizures. A 74-year-old man noticed involuntary movements in the upper extremities when he received isoniazid in pulmonary atypical mycobacteriosis late June 1996. Neurological examination revealed myoclonic jerking in the upper limbs. Although isoniazid treatment had been given to November 1996, the myoclonic seizures were spontaneously ameliorated at 2 weeks following the onset. The second administration of isoniazid was started for the exacerbation of pulmonary atypical mycobacteriosis in February 13, 1999. Immediately he developed myoclonic seizures in the upper extremities. Neurological examination showed myoclonic jerking in the upper extremities and cerebellar ataxia in the lower extremities. These neurological deficits were naturally improved within a week. Brain MRI suggested multiple lacunar infarction. EEG showed slow theta and sharp waves. Somatosensory evoked potential (SEP) revealed giant potentials. There was no renal or liver dysfunction. Serum vitamin B6 levels were decreased slightly. The metabolic studies of serum and cerebrospinal fluid isoniazid concentrations demonstrated slow inactivation of this agent. After the end of isoniazid treatment, EEG and SEP were normal. The results of these studies indicated that slow inactivator of isoniazid and its epileptogenic effects might contribute to the pathogenesis of myoclonic seizures in our patient.