Daily arginine-vasopressin (AVP) excretion was determined by radioimmunoassay in 60 consecutive cases of untreated lung carcinoma. Control excretion was 61 +/- 34 (SD) in 41 healthy subjects and 50 +/- 38 ng/24 h in 10 patients with chronic lung diseases. Overall 20 out of the 60 cases of lung carcinoma presented with abnormally elevated urinary AVP; In the group with anaplastic oat-cell carcinoma, 15 of 23 had elevated urinary AVP with a mean of 370 +/- 331 (SD) ng/24 h if 2 cases with extremely high values of 11 100 and 55 300 ng/24 h respectively are excluded. None of the 9 patients with large-cell carcinoma had elevated urinary AVP, while only 3 of the 19 cases of epidermoid carcinoma and 2 of the 9 cases of adenocarcinoma had high urinary AVP, with means of 127 +/- 8 and 125 +/- 12 ng/24 h respectively. Plasma osmolality and sodium correlated inversely with AVP excretion. However, only 10 of 23 patients with increased urinary AVP had decreased plasma sodium, although one became hyponatremic 9 weeks later. In one patient AVP excretion normalized after radiotherapy. Plasma renin activity and urinary aldosterone were usually low when urinary AVP was high. Two cases with elevated plasma luteotrophic hormone and another with elevated plasma ACTH, all three presenting with oat-cell carcinoma, were found;