The aim of this investigation was to quantify dissemination of Mycobacterium tuberculosis infection in patients with pulmonary tuberculosis and to show the pattern of eradication during treatment. The study is based on 98 out of the 113 patients with pulmonary tuberculosis who died during their admission to hospital in the Municipality of Copenhagen from 1963 to 1971. These patients had cultures for M. tuberculosis performed from different organs at autopsy: 78% treated <=100 days had dissemination of bacteria, cultured with decreasing frequency in the lungs, spleen, liver, and kidneys, respectively. In comparison, 23% treated >100 days had dissemination of bacteria, among which 50% occurred in patients with records of poor treatment compliance, 14% in patients with good treatment compliance. 81% of all patients had at least one chest x-ray judged to be without a miliary pattern. This study emphasizes that M. tuberculosis is often disseminated to organs other than the lungs in severe pulmonary tuberculosis. Eradication of bacteria in these organs can take several months. This observation adds to our understanding of the natural history of tuberculosis: M. tuberculosis is a resilient organism that can adapt to a wide variety of environmental conditions.