The value of ferrokinetic studies for determining the prognosis of patients with aplastic anaemia was assessed 17 patients. Plasma 59Fe clearance, red cell 59Fe incorporation (at 10 days), sacral and hepatic 59Fe uptake (at 6,0 h, measured by surface counting), and 59Fe bone marrow scanning were compared with peripheral blood pancytopenia and marrow cellularity on aspiration and biopsy. In this limited study no statistically significant correlation could be found between ferrokinetic determinations and other criteria of bone marrow failure, although there was a tendency for plasma 59Fe clearance and sacral 59Fe uptake to show some correlation with marrow cellularity on biopsy and with degrees of reticulocytopenia, thrombocytopenia and neutropenia. Ferrokinetic measurements did not differentiate between patients with favourable and those with poor prognoses.