We have presented 73 patients (48 adults and 25 children) with microbiologically documented M. bovis infections identified over the 12-year period from 1980 through 1991. Epidemiologic investigation of these patients revealed that the majority (80%) were of Hispanic origin. The non-Hispanic patients either had traveled extensively outside the United States, were born in the United States during its endemic period or in other countries with endemic bovine tuberculosis, or were exposed to a close relative with a positive PPD and known exposure to M. bovis. For Hispanic patients, the presence of reactivation disease in adults and primary disease in children indicate that this mycobacterium remains endemic in Mexican beef and dairy herds, a position supported by United States monitoring of Mexican cattle transferred across the border. Our review of the historical and contemporary efforts to eradicate this animal and human pathogen from the livestock industry in the United States and abroad shows that the implementation of similar methods could be effective in Mexico. The detailed presentations of selected patients and summaries of the clinical manifestations in the remainder of our 73 patients reveal striking similarities to historical accounts and to more contemporary studies of reactivated disease in England. Although M. bovis infections are still expressed predominantly in extrapulmonary sites (cervical and mesenteric nodes, the peritoneum, and the GU tract), as many as 50% of adult patients will present only with pulmonary disease. Underlying immunosuppressive disorders were particularly prominent in adults with extrapulmonary disease. For example, HIV positive patients accounted for 12 of 48 adults and 1 adolescent patient in our series. Overall, M. bovis infections accounted for almost 3% of all tuberculous disease reported in San Diego County during the study period. The intrinsic resistance of M. bovis to PZA could threaten the response of patients with bovine tuberculosis to the short-course chemotherapeutic regimens now recommended by the CDC and the American Thoracic Society. We strongly recommend continued surveillance for this forgotten pathogen because the importation of Mexican cattle, the migration of Hispanic immigrants from border areas to the United States interior, and the persistence of extrapulmonary disease in immunocompetent and HIV-infected United States citizens assure its persistence in this country.