A case of epilepsy with myoclonic absences is reported. A boy, 3 years and 8 months old, had the first attack at the age of 1 year and 8 months. He was mentally retarded, but had no evidence of organic brain damages. He had been said to have "absence" at another hospital for 2 years until he was referred to our hospital. The attack was characterized clinically by sudden loss of consciousness accompanied with rhythmical massive myoclonias. The ictal EEGs showed synchronous diffuse 3 c/s spike-wave discharges. There was a strict and constant relation between spike-wave discharges and the myoclonia. The polygraphy recording (EEG and EMG of various muscles) was very helpful for the diagnosis of epilepsy with myoclonic absences. We think that epilepsy with myoclonic absences should be considered in any case of "absence" with concomitant 3 c/s spike-wave discharges which is resistant to appropriate treatment, or is accompanied with mental retardation.