Urinary ferritin levels were measured by a “2-site” immunoradiometric assay in normal volunteers and in patients with various hematologic disorders. The mean urinary ferritin concentration in normal subjects averaged 2.2 microgram/liter, only 3% of the serum ferritin level. Elevated urinary ferritin levels averaging 45 microgram/liter were observed in patients with hematologic malignancies, but there was a proportional increase in serum ferritin so that the urinary level still averaged only 7% of the serum value. The highest urinary ferritin values (mean 170 microgram/liter) were associated with chronic hemolytic anemia, and in these patients, urinary ferritin rose disproportionately in relation to the serum, averaging 82% of it. This higher urinary level apparently reflects increased ferritin in renal tubular cells due to glomerular filtration of unbound hemoglobin, a mechanism that is supported by a highly significant correlation between urinary ferritin and serum haptoglobin levels. In normal subjects and in patients with malignancy, the source of urinary ferritin appears different, since a highly significant correlation was observed between urinary ferritin and reticuloendothelial iron stores as measured by serum ferritin or total iron-binding capacity. In this setting, the most likely source of urinary ferritin is the iron contained in renal tubular cells, which is apparently in equilibrium with body iron stores.