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A 68-year-old woman, who had not traveled outside of western Wisconsin, was hospitalized after 4 weeks of chills, fevers, myalgias, neuralgias in her right arm, and pain in the right upper quadrant of her abdomen. Physical examination revealed hepatosplenomegaly, and laboratory studies showed anemia, thrombocytopenia, increased aspartate transaminase level, and microscopic hematuria. Wright's stain of a blood smear revealed intraerythrocytic organisms consistent with Babesiaspecies. A polymerase chain reaction of whole blood specimens along with an increased serologic titer confirmed the diagnosis of Babesia microti.Indirect immunofiuorescent antibody serology and Western blot analysis revealed a simultaneous infection with Borrelia burgdorferi.Coinfection with B. microtiand B. burgdorferimay occur in endemic areas where both organisms are carried by the same tick vector, Ixodes scapularis.The intensity and duration of illness seem to be greatest in patients with concurrent infection.