We report a case of a 13-year-old male with trisomy 21 in Southwestern Ontario, Canada, who presented with bilateral pneumonia, pericardial effusion, and peripheral oedema. The pericardial effusion did not respond to standard treatment options. Evaluation revealed severe dietary restriction, consistent with kwashiorkor. Hospital course was complicated by severe hypoalbuminaemia, hypocalcaemia, hypomagnesaemia, and hypophosphataemia. The pericardial effusion and other findings resolved gradually upon slow introduction of a well-balanced diet and adequate caloric and protein intake. Kwashiorkor is an unusual cause of pericardial effusion and can be overlooked especially in developed countries. It is a type of protein–calorie malnutrition often seen in children of impoverished countries and famine. It is a result of insufficient protein intake in the context of adequate caloric intake. Pericardial effusion not responding to usual treatment is a challenge, and other aetiologies must be considered. Malnutrition is often underdiagnosed or misdiagnosed in developed countries with devastating outcomes if unrecognised. This makes it imperative to consider this diagnosis, recognise potential risk factors, and be prepared to accurately assess overall nutritional status.