The purpose of the study reported here was to determine echocardiographic findings in naturally heartworm-infected dogs, and evaluate these findings to use in the diagnosis of adult worms in the pulmonary artery. Sixteen male dogs (8.8%) of the 180 dogs in our study, weighing 7.5-42.0 kg, ranging in age from 1 to 8 years, were determined to be positive for Dirofilaria immitis according to the results of membrane filtration and antigen ELISA tests. Dogs identified with adult heartworms in their blood were evaluated by parasitological. The mean worm burden in infected dogs was 4730 +/- 5479 (min: 8, max: 13.339) per ml of blood. Two-dimensional echocardiography showed a various number of heartworm echoes in the pulmonary artery in infected dogs. No worm was detected in right atrium. Various pulsed Doppler wave patterns were identified, and we classified them into 4 different types according to their pattern of wave dispersion. In conclusion, echocardiographic detection of worms in this location is strongly suggestive of heartworm disease.