Hyperlipidemia is common during contemporary treatment of childhood acute lymphoblastic leukemia and may increase risk of osteonecrosis, thrombosis, and possibly acute pancreatitis. Marine fatty acids found in fish oil decrease levels of triglycerides and possibly total cholesterol in hyperlipidemic patients. This prospective pilot study provided fish oil for 83 days to seven children undergoing acute lymphoblastic leukemia treatment. On average fish oil was consumed 74% of the intervention period. Further, we found significant lower levels of triglycerides (P = 0.016) and total cholesterol (P = 0.027) compared to 22 historical controls, although correction for one extra PEG-asparaginase dose reduced the level of significance. However, the findings indicate that fish oil may alleviate development of hyperlipidemia during acute lymphoblastic leukemia treatment. Randomized controlled trials are warranted to confirm these findings and to investigate the potential effect of fish oil supplements on development of severe adverse events, including osteonecrosis, thrombosis, and acute pancreatitis.