Partial cor triatriatum sinistrum is a rare congenital heart disease and is usually considered for surgery in symptomatic patients. We describe three cases of partial cor triatriatum sinistrum, two cases of successful percutaneous balloon dilatation, and one case of conservative treatment in a patient without symptoms. The first case describes a female patient with progressive dyspnoea on exertion in which the membrane was dilated with a balloon to relieve symptoms. The second case is about a young patient with chronic congestion of the right lung because of a partial cor triatriatum sinistrum which was also treated with dilatation. The third case describes a male patient with an incidental finding of a cor triatriatum sinistrum who did not experience any symptoms and was treated conservatively. Percutaneous balloon dilatation is a safe and effective alternative to surgery in selected cases of (partial) cor triatriatum.