Abstract: In this study, attempts were made to study the clinico-pathologic findings in dogs with D. immitis infestation in order to more accurately diagnose the infestation. One hundred and twenty two stray dogs in Garmsar area (central part of Iran) from March to November 2006 were examined for infection to D. immitis. Blood microfilaria were observed in 18 samples (14.75%): microfilaria of Dirifilaria immitis in 15 samples (12.29%), microfilaria of Dipetalonema reconditum in 2 samples (1.64%) and mixed infection in 1 sample (0.82%). Then, five dogs with dirofilariasis and five other dogs with negative results for parasite were selected as control group. Hematological and serum biochemical values were assessed in these dogs. Necropsy of infected dogs was also performed and tissue samples were taken from the heart, lung, liver and kidney for histopathological examination. The assessment of hematological parameters showed a mild anemia in dogs with D. immitis infection, with a significant decrease in the number of red blood cells and an increase in number of eosinophil. However, there was no significant difference between infected and non-infected dogs. The infected dogs showed a higher level of aspartate aminotransferase (AST) activity. In necropsy, many pathological lesions were found in sampled tissues, but those are not pathogonomic for dirofilariasis.