6-MP is the backbone of treatment of children diagnosed with acute lymphoblastic leukaemia. Severe hypoglycemia induced by 6-mercaptopurine (6-MP) is a very rare occurrence. Hyperinsulinism is observed in several case reports on literature review. A 10-year-old boy presented with severe symptomatic hypoglycemia that was eventually found to be associated with 6-mercaptopurine treatment. Fasting sample revealed elevated serum insulin, C-peptide, cortisol with ketosis. His liver transaminases were raised. He was able to fast longer, with marked reduction of serum insulin, C-peptide and liver transaminases after stopping 6-MP upon serial fasting tests. No hypoglycemia symptoms reported following reduction of 6-MP and split dosing. This case has verified the evidence of the mechanism by which 6-MP causes hypoglycemia in a small number of individuals through hyperinsulinaemia. This is reversible after stopping 6-MP and adjusting its dosing.