We report a rare and interesting case of mixed infection with Leishmania infantum and Dirofilaria immitis associated with the incidental finding of microfilariae in the bone marrow of a 9-year-old, intact, male Bullmastiff which was seropositive to L. infantum. Clinical signs showed progressive weakness, pale mucosae membranes, and a very low body condition score. Laboratory abnormalities included moderate, normocytic, normochromic, non-regenerative anemia; mild leukocytosis, neutrophilia, monocytosis, and eosinopenia; low platelet count; elevated C reactive protein; mild hyperkalemia, hypoalbuminemia, and hyperbeta-2-globulinemia; and a low A/G ratio. Hypoadrenocorticism, euthyroid sick syndrome, and alteration in the fibrinolytic phase of hemostasis were also detected. Microfilariae were incidentally found in bone marrow cytology aspirate in the absence of clinical features indicative of co-infection with D. immitis. PCR confirmed the identification of the Dirofilaria species. It is assumed that the microfilariae may have left the microcirculation and migrated to bone marrow tissues by crossing the vessel wall. To the best of our knowledge, only one such case has been previously reported in dogs.