Serum iron levels were determined in 16 patients with AMI at different days following the event and in five patients either treated with steroids or suffering from diseases acting as stress factors. Patients with AMI showed a marked decrease in serum iron concentrations, the lowest value being recorded at day 3 after the event; serum iron returned to normal level at day 12. These alterations in iron concentration were not accompanied by changes in the 24 hour urine excretion of iron. On the other hand, a significant shortening of the half-time 59Fe plasma clearance and an early increase in 59Fe incorporation into the red blood cells were found immediately after the event. These values returned to normal in all but one patients, who had in addition polycythemia. In the five patients comprising the control group, serum iron, iron excretion in urine, and ferrokinetics were within the normal limits at all times. Since in AMI patients the 11-OHCS values were found to be significantly higher on the day following the infarction than on day 15 after the event, it is suggested that the decrease in iron concentration in these patients may be connected with changes in plasma 11-OHCS levels.