The effect on urinary iron excretion (UIE) of vitamin C administered orally 2 h after the start of an 8-hour desferrioxamine (DF) i.v. infusion was studied in 12 patients with untreated idiopathic hemochromatosis (IH). Mean +/- SEM basal UIE of 324.6 +/- 84.6 micrograms/24 h increased after a 1-gram i.v. DF infusion to 8,778.5 +/- 1,191.4 micrograms/24 h; when vitamin C 1 or 2 g were added to DF i.v. infusion, there were further increases to 11,241.5 +/- 1,486.1 (p less than 0.01) and 13,531.2 +/- 1,697.2 micrograms/24 h (p less than 0.05 versus the last value), respectively. Basal UIE did not significantly increase after oral vitamin C administration alone. No side effects were observed.