Plasma calcitonin and 6-oxo-Prostaglandin-F1 alpha (6-oxo-PGF1 alpha), one of the stable metabolite of prostacyclin, were determined in patients with malignant and non-malignant diseases of the lung. 11 out of 14 patients with small cell carcinoma and only 3 out of 17 patients with other histological types of lung cancer had abnormally elevated plasma calcitonin levels. 6-oxo-PGF1 alpha levels were significantly higher in patients with different types of lung cancer, compared to a control group with non-malignant lung disease. Combining the results of calcitonin and 6-oxo-PGF1 alpha measurements led to improved specificity and efficiency for the correct differentiation between small cell and non-small cell carcinoma of the lung; the predictive value for the diagnosis of small cell carcinoma approached 90%.