The plasma magnesium (Mg) concentrations of 582 unselected diabetic outpatients and 140 control subjects were measured by atomic absorption spectrophotometry. Mean plasma Mg (+/-S.D.) was significantly lower in the diabetic patients (0.737 +/- 0.071 mmol/l) than in the control subjects (0.810 +/- 0.057 mmol/l), and 146 (25%) diabetics had values below those of all control subjects except one. Plasma Mg correlated best with clinic blood glucose concentration (r = -0.32, p less than 0.001) and other significant associations were observed with glycosuria, age, sex, insulin therapy and biguanide therapy. Although its significance is unclear, hypomagnesaemia could conceivably predispose to ischaemic heart disease in diabetes.