Osteoporosis is the most commonly encountered metabolic bone disease, and metabolic bone-disease clinics have been established to assist in the diagnosis and treatment of uncommon causes of low bone-mineral density. Hypophosphatemia leading to metabolic bone disease may be encountered, and an understanding of phosphate homeostasis can aid in the diagnosis. Two cases of hypophosphatemia leading to low bone-mineral densities were seen at the University of Alabama at Birmingham Osteoporosis Clinic. We developed a diagnostic algorithm, and the laboratory values of each patient were tested with the algorithm. The algorithm, incorporating the use of a spot urine phosphate and spot urine creatinine level at the time of initial serum metabolic profile evaluation, accurately determined the cause of hypophosphatemia in each case.