In March 2004, a U.S.-born boy aged 15 months in New York City (NYC) died of peritoneal tuberculosis (TB) caused by Mycobacterium bovis infection. M. bovis, a bacterial species of the M. tuberculosis complex, is a pathogen that primarily infects cattle. However, humans also can become infected, most commonly through consumption of unpasteurized milk products from infected cows. In industrialized nations, human TB caused by M. bovis is rare because of milk pasteurization and culling of infected cattle herds. This report summarizes an ongoing, multiagency investigation that has identified 35 cases of human M. bovis infection in NYC. Preliminary findings indicate that fresh cheese (e.g., queso fresco) brought to NYC from Mexico was a likely source of infection. No evidence of human-to-human transmission has been found. Products from unpasteurized cow's milk have been associated with certain infectious diseases and carry the risk of transmitting M. bovis if imported from countries where the bacterium is common in cattle. All persons should avoid consuming products from unpasteurized cow's milk.