The safety of tumor necrosis factor (TNF) inhibitors has been demonstrated for over two decades. However, their effects on cardiovascular function in patients with rheumatic diseases remain controversial, and conclusions are additionally hampered by the cardiovascular complications inherent in such diseases. We present two 15-year-old patients diagnosed with ankylosing spondylitis and juvenile idiopathic arthritis classified as polyarthritis with positive rheumatoid factor, respectively. Soon after treatment onset with adalimumab and etanercept, respectively, they developed myocardial inflammation leading to heart failure. Their condition improved upon treatment discontinuation and onset of secukinumab and tocilizumab, respectively. A thorough literature search revealed that these are the only cases of heart failure reported to date after anti-TNF treatment in adolescents with rheumatic diseases. Although cardiovascular adverse effects seem to be very rare in this population, even atypical symptoms of cardiac failure should not be ignored, and cardiac function should be closely monitored when administering anti-TNF-α.