Background: Diazoxide is widely used to manage congenital hyperinsulinism and is generally well tolerated. Pericardial effusion is not a recognized side effect of diazoxide, apart from 2 single case reports. Case Description: Three patients with congenital hyperinsulinism developed pericardial effusion at the ages of 7 weeks, 8 months, and 17 years. The duration of diazoxide treatment (10–15 mg/kg/day) was 6.5 weeks, 5 months, and 17 years, respectively. There was no evidence of fluid overload or significant other cardiac anomaly. The 7-week-old patient presented with signs of cardiac failure, was treated with diuretics, and the effusion resolved after cessation of diazoxide. The 8-month-old patient required emergency subxiphoid drainage of the effusion due to hemodynamic compromise. The pericardial fluid had high numbers of polymorphonuclear cells, but did not grow any organisms, and histology showed non-specific chronic reactive changes; the effusion did not recur after cessation of diazoxide. The 17-year-old patient presented with atrial fibrillation, was treated with beta blockade and colchicine, and continues on diazoxide with monitoring of the effusion by ultrasound. Conclusion: Patients on long-term diazoxide treatment may be at risk of pericardial effusion, the timing and significance of which is unpredictable. The duration of diazoxide treatment before presentation of pericardial effusion varied in our patients from weeks to years. We advise serial echocardiography 1–2 months after commencement of diazoxide and annually thereafter.