A group of 17 patients had proximal diabetic neuropathy characterized by abrupt onset of asymmetric pain and weakness. Fourteen patients had unilateral onset that later involved the other extremity in 3 days to 8 months. All patients reported stepwise or steady progression during 2 to 18 months that was documented during serial examinations. In 16 patients, both proximal and distal muscles were involved. Sural nerve biopsy specimens demonstrated multifocal variability in nerve fiber density manifesting as nonrandom fiber loss between and within fascicles compared with age-matched controls. These findings demonstrate that patients may have a rapidly evolving course of proximal diabetic neuropathy followed by continued progression for many months and emphasize the overlap between proximal diabetic neuropathies of presumed different types. Our cases and others cast doubt on notions supporting two distinct types of proximal diabetic neuropathies represented by the rapid evolution of asymmetric weakness on an ischemic basis, in contrast to a more slowly progressive condition of metabolic pathogenesis.