We describe five patients with previous poliomyelitis who developed multiple sclerosis (two laboratory supported definite and three clinically definite). The initial symptoms of functional deterioration developed a mean of 30 years following poliomyelitis. Initial functional deterioration was due to progressive limb weakness or impaired mobility and in three cases this led to an initial diagnosis of post-polio muscular atrophy. The clinical diagnosis became apparent with the subsequent development of characteristic clinical features, including optic nerve, brainstem, cerebellum, and spinal cord involvement. The occurrence of multiple sclerosis in these patients emphasises that late functional deterioration may be apparently unrelated to previous poliomyelitis. Furthermore, characteristic clinical features may be masked by the severe pre-existing neuromuscular and orthopaedic impairment leading to diagnostic delay and confusion with the ill-defined clinical syndrome of progressive post-polio muscular atrophy.