Thirteen patients with adult-onset vitamin E deficiency due to fat malabsorption were investigated clinically and electrophysiologically. These patients had slightly or moderately decreased serum vitamin E (1.7-4.8 micrograms/ml, normal less than 6.0) or vitamin E/cholesterol ratio (0.21-0.31 mg/g, normal less than 0.35). Only one patient had typical neurological manifestations of vitamin E deficiency, which improved with supplementary vitamin E. The pathological findings in this patient were also compatible with vitamin E deficiency. This patient had poorly controlled diabetes mellitus due to advanced chronic pancreatitis. Reviewing previously reported cases of vitamin E deficiency with diabetes mellitus in chronic pancreatitis, the duration of deficiency until the onset of symptoms was shorter than in those cases without complications. Although adult patients with early, slight deficiency of vitamin E are generally asymptomatic, patients with diabetes mellitus tend to have early neurological symptoms. The vitamin E tolerance test should be used, because even in some patients with vitamin E deficiency due to malabsorption, the deficiency can be overcome by large oral doses of vitamin E.