We describe 4 siblings with spastic paraparesis and peroneal amyotrophy who were prone to severe pain and painful dysesthesias, tetraparesis and pyramidal signs during pyrexial episodes of variable etiology. These symptoms cleared almost completely in 10-20 days. Nerve conduction velocity was reduced more markedly during the spells of fever. Muscle biopsy specimen was normal. Some transient functional disturbance of membrane equilibrium of the nervous pathways of both central and peripheral nervous systems was probably responsible for the attacks during pyrexial episodes.