Acute lymphoblastic leukemia (ALL) is the most common malignant disease among children. Treatment results have improved over time due to intensive risk-adapted therapy and the 5-year survival rate is now above 90%. However, the burden of therapy has increased proportionally. Many children develop serious acute and chronic side effects, which impact on the patients expected lifespan and impair their quality of life as a result of therapy. Treatment with PEG-asparaginase and dexamethasone increases the levels of triglycerides and total cholesterol. Consequently, the incidence of hyperlipidemia is high during initial ALL therapy. Studies have suggested that hyperlipidemia is a risk factor for development of osteonecrosis, thrombosis and possibly acute pancreatitis. Long-chained marine omega-3 fatty acids, found in fish oil, decrease levels of triglycerides and total cholesterol in hyperlipidemic patients. Due to the high survival rate, it is of great interest to develop methods to reduce treatment related toxicities. The investigators hypothesise that daily intake of fish oil will prevent development of hyperlipidemia during ALL treatment phases with dexamethasone and PEG-asparaginase compared to placebo and that fish oil intake may reduce the incidence of severe adverse events related to ALL treatment.