 Preliminary data from this institution suggest that imatinib, likely by inhibiting platelet derived growth factor receptor PDGFR, inhibits bone formation and resorption in a high percentage of patients with either chronic myelogenous leukemia CML or gastrointestinal stromal tumorsGIST.1 Some, but not all, patients taking imatinib developed hypophosphatemia but the effect on bone, as measured by markers of bone synthesis and metabolism, was seen in some patients with normal phosphate levels as well. Marked urinary phosphate wasting with elevated levels of parathyroid hormone was seen in nearly all patients. The effect of imatinib on bone may be dose-related. Patients with hypophosphatemia were routinely started on oral phosphate replacement, but follow up determinations of urinary phosphate wasting were not performed. 