Previous studies have shown that zinc levels in erythrocytes are significantly elevated in patients with multiple sclerosis (MS). To examine the correlation between erythrocyte Zn levels and disease activity, we measured erythrocyte Zn levels longitudinally. Levels were dramatically decreased during a clinically documented exacerbation of MS. To determine the localization of increased Zn levels in MS erythrocytes, we employed standard techniques for the isolation of nonhemoglobin erythrocyte membrane ghosts. Patients with MS had three times more Zn in ghost material than did controls. Chloroform–methanol extraction in erythrocyte ghosts followed by determination of Zn levels indicated that most of the membrane-bound Zn was associated with the lipid-soluble fraction. Non-lipid-associated Zn and total membrane protein concentration were similar in MS and control samples. Results suggest that mechanisms which govern cellular availability, compartmentalization of Zn, or the binding of Zn to cell surface membranes may be altered in patients with MS, and that these mechanisms vary with disease activity.