We hypothesize that there are post-operative, non-surgical risk factors that could be modified to prevent the occurrence of chylothorax, and we seek to determine those factors. Retrospective chart review of 285 consecutive patients < 18 years who underwent cardiac surgery from 2015 to 2017 at a single institution pediatric intensive care unit. Data was collected on patient demographics, cardiac lesion, surgical and post-operative characteristics. Primary outcome was development of chylothorax. Of 285 patients, median age was 189 days, median weight was 6.6 kg, 48% were female, and 10% had trisomy 21. 3.5% of patients developed upper extremity DVTs, and 8% developed chylothorax. At 24 h following surgery, a majority were in the 0–10% fluid overload category or had a negative fluid balance (63% and 34%, respectively), and a positive fluid balance was rare at 72 h (16%). In univariate analysis, age, weight, bypass time, DVT, arrhythmia, and trisomy 21 were significantly associated with chylothorax and adjusted for in logistic regression. Presence of an upper extremity DVT (OR 49.8, p < 0.001) and trisomy 21 (OR 5.8, p < 0.001) remained associated with chylothorax on regression modeling. The presence of an upper extremity DVT and trisomy 21 were associated with the development of chylothorax. Fluid overload was rare in our population. The presence of positive fluid balance, fluid overload, elevated central venous pressure, and early initiation of fat containing feeds were not associated with chylothorax.