EFFICACY: Iron stores will be measured serially in each patient. Measurements will be performed prior to the start of phlebotomy, and at 6 months and 12 months following the start of the series of 12 phlebotomies. These evaluations will be undertaken regardless of the number of phlebotomies which the patient actually undergoes. Iron stores will be estimated by measuring serum ferritin and transferrin saturation levels. Total body iron will be estimated from hepatic and cardiac iron concentration as measured by magnetic resonance imaging (MRI). Gandon et al. (12) described a non-invasive technique using MRI to measure hepatic iron stores. Iron is a paramagnetic substance which causes local magnetic field inhomogeneities leading to dephasing and signal loss in MRI. Gradient echo sequences are most susceptible to their effects because they do not use a 180° refocusing pulse, unlike conventional spin-echo sequences. Gandon et al. used multiple gradient echo sequences, compared the signal in liver to adjacent muscle and used this ratio to correlate with hepatic iron levels measured on tissue biopsy samples using spectrophotometric analysis. Multiple sequences were used because the nomogram comparing the L/M signal ratio is linear over only a small concentration of tissue iron.