Ehrlichiosis is the multiorgan infectious disease caused by small, intracellular rickettsias from the genus Ehrlichia. These microorganisms are known as an etiologic factor of infections world wide in humans and in different species of animals. Dog ehrlichiosis can be caused by several species of Ehrlichia attacking different groups of blood cells, but most often an infection by Ehrlichia canis is diagnosed with special relation to monocytes. A vector for E. canis are Rhipicephalus sanguineus and Ixodes ricinus, commonly occurring in Poland. Disease caused by E. canis is known as Canine Monocytic Ehrlichiosis (CME). The disease most often has an asymptomatic course which can, in favourable circumstances, run into acute or chronic forms. The acute form of CME proceeds usually with fever, apathy, weakness and accompanying respiratory symptoms, lameness and disturbances in blood coagulation. In laboratory examinations thrombocytopenia, anemia and leucopenia are ascertained. The chronic form of CME proceeds among gentle, unspecific symptoms which may last even 5 years. The CME diagnosis is difficult and often demands parallel different diagnostic methods. A medicines of choice in the ehrlichiosis treatment are antibiotics from the group of tetracyclines, given at least for 28 days. They are largely efficient during treatment of the acute CME, causing the quick improvement. Instead, in the case of chronic form, answer for treatment can be weak, and cases of resistance to antibiotics ave known.