The ectopically produced polypeptide hormones ACTH, ADH, and calcitonin were investigated as tumor markers in patients with small-cell carcinoma of the lung (SCC). Plasma ADH concentrations were evaluated separately as well as in relation to concomitantly obtained plasma osmolality levels. No significant nor consistent changes of marker concentrations caused by lysis of tumor cells were found immediately after administration of cytotoxic drugs. After tumor regression, plasma ACTH and serum calcitonin concentrations and inappropriate ADH secretion (plasma ADH levels inappropriately high compared with plasma osmolality) became normal in most cases; however, progressive disease was not followed consistently by changes in plasma ACTH concentrations and occurrence of inappropriate ADH secretion. Contrary to this, among 12 patients with disease progression, serum calcitonin levels increased in ten patients and plasma ADH levels increased in 11 patients. In most cases, however, these changes were only moderate, and serum calcitonin concentrations were found to be increased after tumor regression in patients who had normal pretreatment levels. It is concluded that decisions on treatment of patients with SCC cannot exclusively be based on changes in the concentrations of the polypeptide hormones that might be of ectopic origin.