We studied 5 cases of syphilitic lateral amyotrophic sclerosis. The diagnosis was based on the presence of a lymphocytic reaction in the CSF and positive VDRL and TPHA reactions in both blood and CSF. Clinically, the disease affected the arms in 3 cases and produced paraplegia in 2 cases. The gradual extension of amyotrophy over several months, the diffusion of electromyographic abnormalities and the finding of spinal cord atrophy at myelography and CT suggested a subacute ischemic mechanism with meningo-myelic arteritis involving the anterior horns. After treatment with penicillin G in high doses, the outcome was constantly favourable, with improvement of motor deficit in 4 cases and stabilisation in 1 case in a 5 to 13 years' follow-up.