Human atrial natriuretic peptide (HANP) is a hormone with the physiological characteristics of a regulator of body fluid volume. We studied whether, or not, it is possible to use HANP as a parameter to determine the so-called dry weight (D.W.) in patients on maintenance dialysis. Subjects for experiments included 117 hemodialysis (HD) patients, 18 chronic renal failure (CRF) patients under conservative treatment and 20 normal controls. Plasma HANP level was much higher in HD patients than in controls. In CRF patients treated conservatively, there was no significant correlation between plasma HANP and the degree of renal dysfunction. In HD patients plasma HANP showed a significant positive correlation with CTR (r = 0.408, p less than 0.001), but no correlation with the age or duration of hemodialysis. During hemodialysis, the plasma HANP level fell with the lapse of time significantly (p less than 0.001). In HD patients without complications, plasma HANP level after HD were significantly higher (p less than 0.001) in ones with CTR of 50% or more than with CTR of less than 50%. Plasma HANP may play an important part in regulating the balance of body fluid volume. These findings suggest that plasma HANP is useful as a parameter to determine the D.W. in patients on hemodialysis.