Babesiosis is caused by protozoan parasites of the genus Babesia that infect red blood cells. Babesia infection can range from asymptomatic to life threatening. Clinical manifestations might include fever, other nonspecific influenza-like symptoms, and hemolytic anemia. Babesia parasites in nature usually are tickborne but they also are transmissible via blood transfusion or congenitally. In recent years, reports of tickborne and transfusion-associated cases have increased in number and geographic distribution. However, the lack of a standard case definition hindered the ability of public health authorities to monitor cases and to develop evidence-based prevention and control measures. In January 2011, national surveillance for human babesiosis was begun in 19 jurisdictions (18 states and one city), using a standard case definition developed jointly by CDC and the Council of State and Territorial Epidemiologists. This report summarizes the results for 2011. For the first year of babesiosis surveillance, health departments notified CDC of 1,124 confirmed and probable cases. Cases were reported by 15 of the 18 states where babesiosis was reportable; however, 1,092 cases (97%) were reported by seven states (Connecticut, Massachusetts, Minnesota, New Jersey, New York [including New York City], Rhode Island, and Wisconsin). Cases were identified in persons aged <1-98 years; 57% were in persons aged ≥60 years. Among patients for whom data were available, 82% (717 of 879) had symptom onset dates during June-August. Ongoing national surveillance using the standard case definition will provide a foundation for developing evidence-based prevention and control measures to reduce the burden of babesiosis.