Thirty-four children were followed up prospectively for 5 years from the onset of diabetes regarding serum magnesium, zinc and some proteins. Serum magnesium decreased to significantly lower values (0.76 +/- 0.05 mmol l-1) than those in matched controls after 2 and 5 years, with the lowest mean values in diabetic girls. Serum zinc concentration was higher in the diabetic group than in the control children, and again the diabetic girls differed most from the controls. Serum prealbumin was significantly lower in the diabetic patients after 2 and 5 years than in the controls. Serum albumin was also slightly reduced in the diabetic patients, while orosomucoid was normal. These data indicate chronic magnesium deficiency and insufficient liver synthesis of certain serum proteins in diabetic children.