Four patients, three women aged 49, 47 and 74 years, and a man aged 64 years, presented with progressive sensory deficit, pyramidal tract symptoms and postural instability. Tests revealed megaloblastic anaemia and low vitamin B12 levels. Two of the female patients had undergone gynaecological surgery with nitrous oxide anaesthesia, and the male patient had undergone a gastric resection. Subacute combined degeneration of the spinal cord is a neurological disease based on vitamin B12 deficiency. It involves the posterior and lateral columns of the spinal cord, and sometimes the peripheral nerves, the optic nerve or the brain. An MRI scan of the cervical cord revealed abnormalities for three of the four patients. Following parenteral supplementation of vitamin B12, the symptoms and the MRI abnormalities either disappeared or significantly improved. Vitamin B12 deficiency can cause subacute combined degeneration of the cord by interfering with myelin synthesis. As vitamin B12 deficiency is caused by malabsorption in the gastrointestinal tract, oral supplementation is insufficient. It is essential to recognise this treatable disease at an early stage, and not to reject the possible diagnosis if the MRI findings are abnormal. Simple blood tests can lead to the diagnosis and to effective treatment.