The urinary excretion of 59Fe and total iron following intravenous administration of ferrous citrate Fe 59 was studied in a patient with paroxysmal nocturnal hemoglobinuria over a period of 82 days. A diurnal pattern of iron excretion was observed. Urinary fractionation revealed that the majority of total urinary iron was excreted in the sediment as insoluble iron. 59Fe was initially excreted in a soluble form and later shifted to the insoluble form. The majority of the radioactive iron was retained by the body at the end of the study. These data support previous schemes of renal handling of hemoglobin and experimental animal data.