Leishmaniasis caused by Leishmania infantum is an important zoonotic disease transmitted by sand flies with a high prevalence of infection in dogs and cats in regions whereby transmission occurs. Clinical disease is systemic with variable presenting signs and degrees of severity. It affects the skin, lymph nodes, eyes, bone marrow, kidneys, and other organs. The clinical findings in dogs and cats with L. infantum infection are generally similar. Subclinical infection of canines and felines in endemic areas is frequent. Long-term treatment of the disease with allopurinol, or combination of allopurinol with meglumine antimoniate or miltefosine, is needed, and clinical relapse is probable.