In hereditary hemochromatosis (HH), increased intestinal iron absorption leads to the development of iron overload. To examine the abnormal regulation of iron absorption in this disorder, we analyzed mucosal iron kinetics in six patients with HH and in five normal subjects by using a compartmental model of intestinal iron absorption and systemic ferrokinetics. Subjects were given simultaneous oral and intravenous tracer doses of iron 59-labeled citrate and iron 55-labeled transferrin, respectively. Plasma and whole-body radioactive iron levels were then monitored serially during the next 2 weeks, and mucosal iron transport rate constants were estimated by non-linear least-squares fit of the model to these data. Iron absorption was inversely related to serum ferritin concentration in both normal subjects and patients with HH but was higher in relation to serum ferritin level among the latter (p less than 0.0002). Analysis of mucosal iron kinetics demonstrated a similar inverse relationship between the rate constant for mucosal iron uptake and serum ferritin among all subjects combined, but the mean uptake rate constant in patients with HH did not differ from that of normal subjects (p = 0.71). The mean rate constant for incorporation of iron into the mucosal storage pool in patients with HH also was comparable to that of normal subjects (p = 0.94). In contrast, the rate constant for transfer of mucosal iron to the plasma was higher in patients with HH than in normal subjects for any given serum ferritin level (p less than 0.0001), and the transfer rate constant accounted for 87% of the variability in iron absorption among all subjects. We conclude that the increased iron absorption in HH is mediated primarily by an increase in the rate constant for transfer of mucosal iron to the plasma.