The medical records of 91 dogs with heartworm (Dirofilaria immitis) infection were reviewed, and diagnoses were established by using parasitological and immunological methods. Twenty-one animals were asymptomatic (stage I), 57 had mild to moderate clinical signs (stage II), and 13 had the severe form of the disease including right congestive heart failure and the caval syndrome (stage III). Thoracic radiography revealed right ventricular enlargement in 38 of the dogs, pulmonary vascular enlargement in 43, and parenchymal lesions in 27. Only the cardiac and vascular changes were correlated positively with the clinical stages. D. immitis microfilaraemia was detected in 75 of 85 dogs. Occult infection occurred only in eight stage II and two stage III dogs. Thirty-two of the dogs were treated with thiacetarsamide and 39 were treated with melarsomine, and no differences were found in terms of drug efficacy or complication rate; nine stage II dogs suffered pulmonary thromboembolism and one suffered acute liver disease and there were six fatalities. The 50 treated dogs in stages II and III which were followed up for six months all recovered completely. The performance of 38 of 61 working dogs was completely restored, and the performance of another four was partially restored.