The ability of metastatic cells to survive antiapoptotic signals may contribute to the organospecific-spread patterns of clinical metastasis and dormancy. MDA-MB-435 breast cancer cells (435/Bcl-x(L)), which overexpress the Bcl-x(L) gene, were labeled with the luciferase gene and injected orthotopically into homozygous athymic Balb/c (nude) mice to study the metastatic behavior of the breast cancer cells. The overexpression of Bcl-x(L) in tumors increased the overall metastatic burden in mice (bones, liver, kidneys, brain, lungs, and lymph nodes) in comparison with control tumors (435/NEO:luc) during the same time interval (ANOVA, p = 0.005). The principal differences after 110 days were found in bones, which had 1.5 x 10(5) +/- 1.2 x 10(5) tumor cell equivalents (p = 0.03), and lymph nodes, which had 7.0 x 10(6) +/- 6.0 x 10(6) tumor cell equivalents (p = 0.08). The analyses of light production by tissues at different times showed that cells from 435/NEO:luc and 435/Bcl-x(L).luc tumors were detectable in several organs by the second day after intramammary fat pad implantation. Although initially arriving at the target organs in similar numbers, 435/Bcl-x(L) cells developed more metastases than 435/Neo cells, indicating that the Bcl-x(L) gene might have a role in breast cancer dormancy, promoting survival of cells in metastatic foci. Thus, we suggest that overexpression of Bcl-x(L) could counteract the proapoptotic signals in the microenvironment and favor the successful development of metastasis in specific organs.