In the "iron excretion test" , urinary iron excretion after injection of saccharated iron oxide has been reported to be accelerated in relapsing idiopathic iron deficiency anemia. To determine the relevance of urinary iron excretion to clinical factors other than iron metabolism, 15 clinical parameters were evaluated. The serum creatinine level was positively and the serum albumin level was negatively correlated with urinary iron excretion, showing coefficients of r＝0.97,－0.86 respectively, and suggesting a relationship between urinary iron excretion and subclinical abnormalities of kidney function. In experimental studies, the relation of urinary iron excretion to the renal function was examined by administration of iron in various forms to rats. Only saccharated iron oxide was excreted; chondroitin sulfate Fe, Tf-Fe and ferric chloride were not excreted in the urine. Then, iron excretion was examined in iron deficient, iron overloaded and puromycin aminonucleoside (PA)-treated animals. Iron deficient rats did not show any change in urinary iron excretion compared to the controls. Urinary iron excretion was increased in iron overloaded rats, and was further increased in the PA-treated group. These findings suggest that the subclinical abnormality in kidney function leads to the increased urinary iron excretion as a possible factor in the pathogenesis of relapsisg iron deficiency.