We had noted that a number of hospitalized patients showed abnormally low serum albumin levels within a few days of admission, although the albumin had been normal at admission. Since this rapid decline in albumin could not be accounted for on the basis of starvation, we hypothesized that the changes were due to the increase in intravascular fluid volume which normally occurs with assumption of the recumbent position. Since albumin is often a nutritional screening parameter in hospitalized patients, it is important to ascertain the incidence of such profound changes in albumin as well as to identify possible causes for this change. A survey of 34 concurrent hospital admissions showed that 28 out of 34 (82%) patients had a decline in serum albumin within 5 days of hospitalization. These 28 patients had a decrease of 0.5 +/- 0.09 g/dl. Twenty-five additional patients were studied in order to elucidate causative factors. Twenty out of the 25 showed a decrease in serum albumin within 5 days of admission (mean decrease 0.5 +/- 0.05 g/dl). Hemoglobins in these patients decreased by a mean of 1.1 +/- 0.34 g/dl (p less than 0.01), but BUN and uric acid levels did not change significantly. The one factor common to all patients with declining albumin values was change in posture. All patients were ambulatory at the time of the initial albumin determination but were on bedrest for at least 6 1/2 hours before the second determination. The findings indicate that most hospitalized patients have significant changes in serum albumin levels which occur with change in posture. If the albumin level is to be used as a nutritional indicator, the patient's position at the time of phlebotomy is essential for accurate interpretation of results.