The loss of body cell mass (bcm) in senescence and wasting is poorly understood. We now show that the plasma cystine/acid soluble thiol ratio, ie, an indicator of the redox state, is increased in old age and cancer patients and correlated with a decrease in bcm and plasma albumin. A cause/effect relationship was suggested by two independent studies with N-acetyl-cysteine (NAC). NAC caused an increase in the bcm of healthy persons with high plasma cystine/thiol ratios, and treatment of cancer patients with NAC plus interleukin-2 caused an increase in bcm, plasma albumin, and functional capacity. Albumin levels below 680 micromol/L were associated with an increase in body water. Our studies suggest that the shift in the redox state may contribute to the loss of bcm and may provide a quantitative guideline for therapeutic intervention. Treatment of cancer patients with thiol-containing antioxidants may improve the quality of life.