The aim of this study was to determine the prevalence and distribution of canine dirofilariosis from different districts of the Mazovian Province in central-eastern Poland. 462 dogs aged from 1.5–14 years were examined for dirofilariosis. Blood samples were examined for the presence of microfilariae using the Knott method, as well as the method of Kingston and Morton, after centrifugation in haematocrit microtubes in order to determine the intensity of infection as the number of microfilariae in 60 microliters of blood. The species of microfilariae found were determined after staining on the basis of the morphological characters. Samples were also examined using Canine Heartworm Antigen Test Kit SNAP HTWM (IDEXX, USA) that allows the detection of circulating antigens of females of Dirofilaria immitis. The positive samples were examined using multiplex PCR assay for species confirmation. Microfilariae belonging to the species D. repens were found in the blood samples of dogs originating from the city of Warsaw and from 18 districts of Mazovian Province. The mean prevalence of that species observed in the province was 25.8%. The range of intensity counted with the number of microfilariae found in 60 microliters of blood amounted to between 1 – 150; median intensity 9 microfilariae. The highest prevalence, reaching 52.9% and the highest median intensity of infection reaching 28 microfilariae, was found in Radom district. The lowest prevalence, amounting to 4.2%, was registered in Grójec district; however, the lowest median intensity reaching 6 microfilariae was noted in Żyrardów district. The infected dogs had not been outside of Poland, which means that this is an autochthonous infection. Microfilariae and antigens of D. immitis were not found in any examined blood samples. Results of the presented study show that autochthonous dirofilariosis caused by nematodes D. repens commonly occurs in the area of the Mazovian Province.