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Tuberculosis (TB) is a public health concern in correctional facilities. High turnover of inmate populations may preclude timely diagnosis of TB, so that unrecognised transmission may be common.To determine the proportion of inmates with new skin test conversions who had identifiable exposure to diagnosed cases of TB in the correctional system, and to test the hypothesis that source cases of TB may be undiagnosed during incarceration.Maryland Division of Corrections, USA.All inmates whose skin test converted from negative to positive at annual screening.All cases of TB in inmates, diagnosed in the prisons during the relevant time period, were identified. Movements of skin test converters and potential source cases within the prisons were matched. We then matched all inmates discharged from the prison system with all new cases of tuberculosis notified to the Maryland Department of Health & Mental Hygiene tuberculosis registry in 1994. Results: The inmate turnover was 21% per year. Probable exposure to a diagnosed source case was found in 13% of converters, possible exposure in 10% and no exposure in 72%. In a further 5% exposure status could not be determined. We identified four cases of pulmonary tuberculosis notified to the state in 1994, within 3 months of discharge in released inmates, who were not known to have tuberculosis whilst incarcerated.Significant transmission of TB due to undiagnosed index cases may occur in prisons due to high population turnover. New skin test conversions in inmates should be treated as new infection, even in the absence of identifiable exposure to TB.