Macrocytosis, without anaemia, was common in 82 de novo multiple sclerosis patients compared with a similar number of age and sex matched controls. This was an early phenomenon in the course of the disease and was not influenced by the age of the patients nor the duration of the disease. None of the patients proved to have pernicious anaemia, yet the similarity in the geographical and sex distribution as well as the similarity in HLA associations of multiple sclerosis and pernicious anaemia may indicate that both diseases are under similar genetic influence.