A one-year-old female neutered beagle was presented with marked abdominal effusion. Echocardiography showed marked dilatation of the right cardiac chambers, an atrial septal defect and severe tricuspid insufficiency. Systolic pulmonary arterial pressure (sPAP), evaluated by continuous wave Doppler echocardiography, was very high (80 mmHg), with a right to left interatrial shunt. The radiographic images were compatible with widespread pneumonitis. Numerous larvae of Angiostrongylus vasorum were visible on direct faecal examination. The animal was given fenbendazole for 15 days, combined with diuretics, an antibiotic and a vasodilator. Two weeks later, the dog showed a marked improvement. The treatment, except the anthelmintic, was continued for seven weeks and then stopped. At that stage, Doppler echocardiography revealed that the sPAP had returned to normal (20 mmHg) and the interatrial shunt had reversed (left to right). Eighteen months later, clinical and Doppler echocardiographic examinations were normal.