Blood samples from 102 volunteers who believed they suffered from myalgic encephalomyelitis were photographed in a scanning electron microscope at 500x. All identifiable cells were counted and classified on the basis of their shape. The frequency of each cell shape was expressed as a percentage of the total number of cells counted in the sample. The resulting data were compared with that from 52 healthy controls and 99 cases of multiple sclerosis which had been selected randomly by a computer from a panel of 229 cases in a concurrent study. Samples from subjects with myalgic encephalomyelitis had the lowest percentages of normal red cells and the highest incidence of cup forms. The results provide evidence that myalgic encephalomyelitis has an organic cause. Quantitative analysis of red cell shape may assist in the diagnosis of myalgic encephalomyelitis.