In a three years study began from fall 1996, 234 local cats were clinically examined in one of the small animal clinics in Tabriz, east Azarbayejan Province, Iran. Blood smears were examined for blood parasites and for white blood cells differential count. In 25 cats clinical symptoms including inappetence, asthenia, emaciation, chronic coughing, dispnea and vomiting were noticed. These signs could be due to Dirofilaria immitis. For confirmation of heartworm disease, in addition of examination of blood smears stained with Geimsa, blood of cats showing clinical symptoms examined for micorfilariaemia using modified Knott method. The latter showed that two cats harboured 250 and 45 microfilariae of D. immitis in their blood. Eosinophilia (8%) was observed in one of the infected cat. This cat was treated with two intramuscular injection of melarsomine dichlorhydrate (immiticide) as adulticide between the third and fifth lumbar vertebrae at the dose rate of 2.5mg/kg at a 24 hour interval. Two subcutaneous injections of ivermectin (0.05mg/kg as microfilaricide) one and two months after treatment with adulticide were perfomed. Supportive treatments such as antibiotics, aspirin, prednisolon were used. One month after treatment, clinical sings disappeared and no microfilaria was seen in the blood samples tested one and two month after treatment. This is the first report of the existence of D. immitis in cats in Iran.