A 7-year-old boy suffering from X-linked hypogammaglobulinemia and progressive myoclonic encephalopathy is reported. The onset of neurological disturbances is at four years of age with ataxic gait and myoclonic jerks. The EEG shows a progressive slowing of background activity, bilateral diffuse and repetitive, pseudoperiodic, high amplitude slow waves, myoclonic jerks polygraphically documented. The CT-scan shows generalized cerebral atrophy, white matter hypodensity--principally in the frontal regions -, multiple nodular calcifications, also in the basal ganglia. Two years after the onset of neurological signs, the boy is completely bedridden, spastic, dement and blind; the myoclonic jerks persist. Finally the relationship is discussed with both the previously reported patients with the same affection, and with similar progressive encephalopathy in children suffering from A.I.D.S.