The clinical features and investigative findings of 50 patients with chronic progressive external ophthalmoplegia (CPEO) were analysed. The group was found to be clinically, genetically and histologically heterogeneous. With the possible exception of patients with "ophthalmoplegia plus," namely those who in addition to muscular weakness had evidence of central and/or peripheral nervous system abnormality, there was no apparent justification for separating out from among the group patients' subgroups which were distinctive enough to be recognized as syndromes. CPEO therefore seems to represent a number of different degenerative disorders whose common denominator is ophthalmoplegia and for which there are to date no adequate criteria for further classification.