Previously we showed that modified fat breast milk (MFBM) facilitated resolution of post-surgical chylothorax in cardiac infants, but their weight-for-age and length-for-age z-scores declined over the ≥6-week treatment duration. Our aim was to evaluate the growth of infants diagnosed with post-surgical chylothorax and fed according to one of two proactive feeding protocols using MFBM or a high medium triglyceride (MCT)-containing formula (standard of care). In this open-label trial, infants who were receiving >50% of their enteral feeds as breast milk prior to chylothorax diagnosis were randomized to receive their enteral feeds according to one of two proactive MFBM protocols: Target Fortification (n=8), where the protein concentration of defatted breast milk was measured weekly and multi- and single-nutrient modulars were added to provide 3.5g/kg/day of protein; or Higher Initial Concentration (n=8), where defatted breast milk was initially fortified to an energy and nutrient level higher than that of unmodified breast milk (80kcal/100ml; 2.2g/100ml protein). A third nonrandomized group of infants (n=8) received high MCT formula (68kcal/100ml; 2.3g/100ml protein). The intervention lasted for a minimum of 6-weeks after chest tube removal and continued after discharge. Weekly weight, length and head circumference (HC) measurements were completed. At enrolment, there was no statistically significant differences in mean (±SD) weight-for-age (-1.6±0.9, n=24), length-for-age (-1.3±0.8), or HC-for-age (-0.9±1.0) z-scores among groups. Changes in mean weight- (-0.3±0.9, n=23), length- (0.1±0.6) and HC-for-age (0.2±0.6) z-scores did not differ among groups over the treatment period. There was no difference in duration or volume of chest tube drainage across groups. Use of proactive MFBM feeding protocols both resolve chylothorax and support growth in infants following cardiothoracic surgery. ClinicalTrials.gov (NCT02577419).