Objective: To investigate the incidence of early severe hypophosphatemia, and examine the associated clinical factors and outcomes, in extremely low birth weight infants (ELBWI) who received early amino acid administration. Methods: Medical records of 82 ELBWI were retrospectively reviewed. Severe hypophosphatemia was defined as a serum phosphate level <2 mg/dL during the first week after birth. Results: Nineteen ELBWI (23.2%) experienced severe hypophosphatemia. The supplementation of phosphorus was started significantly later in the hypophosphatemia group compared to that in the control group (P=0.036). Small for gestational age infants (SGAI) (P=0.006) and bronchopulmonary dysplasia (P=0.016) were more prevalent in the hypophosphatemia group compared to that in the control group. Conclusion: Early severe hypophosphatemia is common in ELBWI. Late and insufficient supplemen tation of phosphorus and SGAI were associated with severe hypophosphatemia.