Hydropericardium or pericardial effusion (PE) is the accumulation of fluid in the pericardial sac and is often secondary to pericardial diseases or diseases related to the heart. It is unusual with clinically relevant pericardial diseases in cats but pericardial effusion is the most common one. Since PE causes serious symptoms and may lead to death it is important with a quick diagnosis and treatment. PE can be acute and cause cardiac tamponade, which gives severe clinical signs. Cardiac tamponade needs to be treated immediately or else the cat will die from cardiogenic chock. More common is chronic PE where the fluid accumulates during a longer period of time and the pericardial sac stretches.