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In 3 patients with adrenocortical carcinoma the effects of long-term mitotane therapy on the serum levels of three hormone-binding globulins and vitamin D-binding protein were studied. Within the first month of treatment cortisol-binding globulin increased two to three times, in close correlation with sex hormone-binding globulin. The rises in thyroxine-binding globulin and vitamin D-binding protein were considerably less. Elevated cortisol-binding protein appeared to be associated with increased binding of cortisol, whereas the binding of thyroxine and vitamin D remained below normal. Binding proteins returned to normal in 2 patients within a year after mitotane discontinuation. This phenomenon of hormone-binding protein enhancement invalidates the use of total serum hormone levels to monitor the effects of mitotane on endocrine function and could provide an explanation for the increased cortisol substitution requirement during mitotane therapy.