Red blood cell, serum and tissue Mg concentrations in 100 subjects with aged 34-78, with squamous cell carcinoma of the larynx were determined by atomic absorption spectrophotometry. The concentration of this cation in cancerous tissues was found to be about twice as high as in the unchanged homologous tissues of the host. This was accompanied by a systemic deficiency manifesting itself by a decreased Mg concentration in red blood cells, serum and muscles which became more pronounced as the disease advanced, and returned to normal after radical treatment.