A previously healthy young child presented with a large pericardial effusion and cardiac tamponade. The chest radiograph was key to the recognition of the pericardial effusion. Cytologic examination of the pericardial fluid ultimately established the diagnosis of acute monoblastic leukemia in the absence of associated clinical or laboratory findings. The pericardial fluid was vital for leukemic cell classification because the bone marrow has hypocellular and non-diagnostic. This presentation of acute monoblastic leukemia is very rare, and in the three previously reported pediatric cases has been associated either with peripheral blasts or a history of preleukemia. When the cardiac configuration suggests pericardial effusion in a previously healthy young child, the diagnosis of new onset leukemia should be considered.