Bronchioloalveolar lung carcinoma (BAC) is a form of lung cancer for which etiology and pathogenesis are controversial. While squamous cell carcinomas and small cell carcinomas have shown an overall decrease in incidence during the past four decades, BACs and peripheral adenocarcinomas (PAC) have shown exponential increases during this same time period. Some of the distinguishing pathological, biological, epidemiological, and perhaps etiological features of BAC/PAC include its peripheral location, its association with desmoplasia (scarring), its comparatively high female/male ratio, and its high incidence of multifocality. These days there is emerging evidence that stem cells and transformed stem cells are the source and reservoir of many human cancers. Based on the multifocality of BAC/PAC, its penchant for recurring, its penchant for initially responding to targeted therapies and then relapsing and other unique biological features, certainly the applicability of the stem cell hypothesis to human BAC/PAC would appear real and worthy of study. We recently made a startling observation in patients who had received a bone marrow transplant from a different sex donor for non-Hodgkin’s lymphoma or leukemia who years later developed lung BAC/PAC. The BAC/PAC which developed was of donor origin! This could only have been the case if a stem cell derived from the bone marrow of the donor was the cell of origin of the BAC/PAC. We would like to test this hypothesis further in two experimental mouse models, so-called transgenics which express certain genes that cause the mouse to develop BAC/PAC. If our hypothesis is correct and, in fact, a bone marrow-derived stem cell gives rise to BAC/PAC then transplanting the bone marrow from these transgenic mice into normal recipients (which do not normally get lung cancer) will result in the recipient mice getting BAC/PAC. Conversely transplanting bone marrow from normal donors into the transgenic mice whose original bone marrow we destroy by irradiation should result in a decrease or absence of BAC/PAC. If we can demonstrate experimental proof for our hypothesis, we should be able to identify and purify the putative bone marrow stem cell that gives rise to BAC/PAC because they will be marked with the causative transgene.