We have been monitoring a 12-year-old boy (his present age) suffering from selective magnesium (Mg) malabsorption. After ascertaining his Mg status, we attempted to maintain Mg balance through the use of oral supplements. Plasma and erythrocyte Mg concentrations were monitored at bimonthly intervals. However these measurements did not accurately reflect Mg status and we subsequently measured Mg in the following tissues: lymphocyte, tooth, and hair. Levels of Mg in stable tissues such as temporary teeth and lymphocytes, unfailingly revealed a marked deficit in Mg that was only two-thirds of the normal levels found in the control group. The use of these readily-accessible stable tissues can thus obviate the need for muscle or bone biopsy. Unfortunately the level in hair is higher than in healthy subjects, and thus does not constitute an adequate measure of Mg status.