Eight patients with classical motor neurone disease, and eight control patients with neurological disease and matched for degree of wasting, were studied. Both groups had abnormal glucose tolerance, but the patients with motor neurone disease had a significantly lower insulin response both to oral glucose loading and to intravenous tolbutamide. These results suggest that in patients with motor neurone disease there is an impaired synthesis or release of insulin due to islet cell damage. Blood pyruvate and lactate, and cerebrospinal fluid pyruvate, lactate, and citrate, did not differ significantly from the control group. Blood citrate levels were significantly higher in patients with motor neurone disease compared with the controls. Triglyceride levels were raised in patients with motor neurone disease compared to the control patients. This may be secondary to the increased citrate levels.