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After lung injury and damage to the alveolar epithelium, the underlying basement membranes become exposed. Proliferation of type II pneumocytes and their differentiation to the type I phenotype have been considered to be the mechanism by which repopulation of the alveolar epithelium occurs. A growing body of evidence has shown that tissues can be repaired by cells acquired via the circulation. For the lung, bone marrow stem cells have been shown in mice to regenerate epithelium as well as give rise to the expected mesodermal derivatives. We hypothesized that extrapulmonary cells, including those from the bone marrow, can contribute to the reepithelialization of human alveoli. To investigate this, we examined samples of peripheral lung from patients who had undergone cross-gender transplantation of lung or bone marrow. Thus, archival blocks of peripheral lung were analyzed from male patients (surgical samples, n = 8) who had received a lung transplant from a female donor and female patients (postmortem samples, n = 3) who had male bone marrow transplants. In both cases, male cells were identified in the female lungs by Y chromosome in situ hybridization. Male cells could be identified in the alveolar epithelium where, in the better preserved, transplanted lungs, it was possible to show that some had differentiated to type II pneumocytes. In addition, Y chromosomes were found to be widespread in cells of mesenchymal lineage, including macrophages and endothelial cells. Concomitant visualization of Y and X chromosomes, using fluorescence immunolabeling, yielded no evidence of cellular fusion, although the poor quality of the autopsy samples studied meant that the possibility could not be excluded. These observations suggest that, as occurs in rodents, the epithelium of the adult human lung has the capacity to renew itself, using cells recruited from extrapulmonary sources, including the bone marrow. This finding could provide new therapeutic opportunities for a range of pulmonary diseases by providing means to repair the lung and a novel route for gene therapy.