We evaluated serum angiotensin-converting enzyme (ACE) activity and thyroid function to determine the importance as prognostic factors in acute renal failure (ARF). Among 37 ARF patients, 24 survived (recovered group) and 13 died (lethal group). Both serum thyroxine (T<sub>4</sub>) level and ACE activity were lower in ARF as a whole group (n = 37) than normal control group (n = 27, p <0.01). Both serum T<sub>4</sub> level and ACE activity in lethal group (n = 13) were significantly lower than those of the recovered group (n = 24, p < 0.01). Serum ACE activity but not serum T<sub>4</sub> level was significantly related to the prognosis of ARF (p < 0.02, Mantel-Haenszel analysis). This suggests that lowered serum T<sub>4</sub> levels are not a specific indicator of ARF, but lowered ACE activity is significantly related to the prognosis of the patient in ARF. Serum ACE activity seems to be one of several clinically useful prognostic indicators in ARF.