Hypophosphataemia is a metabolic disorder that is commonly encountered in critically ill patients.Phosphate has many roles in physiological functions, thus the depletion of serum phosphate could leadto impairment in multiple organ systems, which include the respiratory, cardiovascular, neurologicaland muscular systems and haematological and metabolic functions. Hypophosphataemia is defined asplasma phosphate level below 0.80 mmol per litre (mmol/L) and can be further divided into subgroupsof mild (plasma phosphate of 0.66 to 0.79 mmol/L), moderate (plasma phosphate of 0.32 to 0.65mmol/L) and severe (plasma phosphate of less than 0.32 mmol/L). The causes of hypophosphataemiainclude inadequate phosphate intake, decreased intestinal absorption, gastrointestinal or renal phosphateloss, and redistribution of phosphate into cells. Symptomatic hypophosphataemia associated withhaematological malignancies has been reported infrequently. We report here a case of asymptomaticsevere hypophosphataemia in a child with acute T-cell lymphoblastic leukaemia.A 14-year-old Chinese boy was diagnosed to have acute T cell lymphoblastic leukaemia (ALL).His serum biochemistry results were normal except inorganic phosphate and lactate dehydrogenaselevels. The serum inorganic phosphate level was 0.1mmol/L and the level was low on repeatedanalysis. The child had no symptoms related to low phosphate levels. The possible causes of lowphosphate were ruled out and urine Tmp/GFR was normal. Chemotherapy regime was started andthe serum phosphate levels started to increase. Hypophosphataemia in leukaemia was attributed toshift of phosphorus into leukemic cells and excessive cellular phosphate consumption by rapidlyproliferating cells. Several reports of symptomatic hypophosphataemia in myelogenous andlymphoblastic leukaemia in adults have been reported. To our knowledge this is the first case ofsevere asymptomatic hypophosphataemia in a child with ALL.