Increased phosphate (P) uptake by the replicating neutrophils during engraftment syndrome has been described to play a role in the development of hypophosphatemia (HP) in bone marrow transplantation patients, and suggested as a measure of neutrophil recovery. Here, the relationship of serum P with engraftment was determined in 56 children who underwent allogeneic bone marrow transplantation (BMT). Uric acid (UA) levels were also analyzed to study the contribution of cytolysis on P levels. HP and hypouricemia (HU) developed at least once in 63 and 57% of patients respectively, before and until day +20 after transplantation. The minimal values of P and UA were observed at day +10 and were significantly lower than the baseline values (p<0.01). The mean neutrophil engraftment was at day +13, following the P and UA nadir by 3 days. In addition there was a significant correlation between P and UA levels (p=0.01). The levels of of both P and UA returned to pretransplant values at day +20. A significant correlation (p<0.05) between platelet engraftment and P levels was also demonstrated. HP and HU seen in pediatric patients undergoing BMT reflects a combination of pathophysiologic mechanisms.