I present to you the first ever case of Sub-mitral Left Ventricular aneurysm following Group A Streptococcus pericardial effusion in a pre-school going age (3-year-old girl) child. She presented to A&E with tachypnoea and tachycardia preceded by 2 weeks history of fevers, sore throat (diagnosed Tonsillitis) and lethargy. A globular heart was noted on X-Ray. Which was diagnosed as a large pericardial effusion on echocardiography. Treated with pericardiocentesis, drain placement and 4 weeks of IV antibiotics; she continued to improve clinically whilst on anti-biotic treament. Routine predischarge echo, 48 h post Anti-biotic stoppage showed a structure behind Left ventrical with flow into it- which was diagnosed as Left ventricular aneursym on Cardiac MRI and surgically repaired with un-eventful recovery. Although, Cardiac complications are relatively rare now-a-days post Streptococcal (Group-A) Tonsillitis in United Kingdom paediatric population, we should be mindful of the potential sequelae and include throat swab in our list of investigations for prolonged, unresolved and previously (mis-)diagnosed “?viral tonsillitis”.