We cared for a patient in whom iron dextran administration interfered with the determination of total serum calcium concentration. An unexpected elevation in serum phosphorus concentration also occurred after the iron dextran infusion. A MEDLINE search from 1966 to present was conducted, and the manufacturer of the iron dextran was contacted for information related to these findings. Several drugs and diseases were found that may decrease serum calcium and increase phosphorus concentrations. We found one anecdotal citation of iron dextran interfering with serum calcium concentrations, but no reports of interference with serum phosphorus concentrations. Doses of iron dextran in excess of 250 mg may cause a false decrease in total calcium concentration more than 4 hours after the infusion is completed. A false increase in serum phosphorus concentrations after the infusion requires further investigation.