Background: We evaluated tolerance of hydrolyzed liquid protein (LP) supplement added to fortified human milk (HM) to optimize protein intake in preterm infants. Methods: A prospective observational study of 31 subjects compared with 31 historic controls, receiving mothers own milk (MOM) and/or donor milk (DM) to assess LP tolerance, growth, and risk for morbidities was conducted. Milk was analyzed for nutrient content. Feeding intolerance, defined as cessation of feedings for ࣙ48 hours, abdominal distension and/or residuals, necrotizing enterocolitis (NEC), and metabolic acidosis were used to assess safety, while weight and head circumference (HC) were used to evaluate growth. Results: LP added to powder-fortified HM had no impact on feeding intolerance and NEC. Biochemical parameters showed no metabolic acidosis: blood urea nitrogen levels (first week: median, 13 mg/dL; interquartile range [IQR], 9-16; last week: median, 13 mg/dL; IQR, 10.3-14; P = .94), bicarbonate levels (first week: median, 26.3 mEq/L; IQR, 24-28; last week: median, 28 mEq/L; IQR, 26.3-29.8; P = .10), and pH levels (first week: median, 7.4; IQR, 7.3-7.4; last week: median, 7.4; IQR, 7.37-7.40; P = .5). Weight and HC were not statistically significant. HM analysis showed lower protein and caloric content, respectively (MOM: 0.88 vs DM: 0.77 g/100 mL; P < .0001 and MOM: 18.68 vs DM: 17.96 kcal/oz; P = .02). Conclusions: Hydrolyzed LP is well tolerated in preterm infants with no difference in growth rates. Clinicians should focus on the need to maximize both protein and energy to optimize growth. (Nutr Clin Pract. 2019;34:450-458) Keywords enteral nutrition; human milk; infant; protein supplement From the