Aquaporin 1 (AQP1) is a plasma membrane water-transporting protein expressed strongly in tumor microvascular endothelia. We previously reported impaired angiogenesis in implanted tumors in AQP1-deficient mice and reduced migration of AQP1-deficient endothelial cells in vitro. Here, we investigated the consequences of AQP1 deficiency in mice that spontaneously develop well-differentiated, luminal-type breast adenomas with lung metastases [mouse mammary tumor virus-driven polyoma virus middle T oncogene (MMTV-PyVT)]. AQP1+/+ MMTV-PyVT mice developed large breast tumors with total tumor mass 3.5±0.5 g and volume 265±36 mm 3 (se, 11 mice) at age 98 d. Tumor mass (1.6±0.2 g) and volume (131±15 mm 3, 12 mice) were greatly reduced in AQP1−/− MMTV-PyVT mice (P< 0.005). CD31 immunofluorescence showed abnormal microvascular anatomy in tumors of AQP1−/− MMTV-PyVT mice, with reduced vessel density. HIF-1α expression was increased in tumors in AQP1−/− MMTV-PyVT mice. The number of lung metastases (5±1/mouse) was much lower than in AQP1+/+ MMTV-PyVT mice (31±8/mouse, P< 0.005). These results implicate AQP1 as an important determinant of tumor angiogenesis and, hence, as a potential drug target for adjuvant therapy of solid tumors.—Esteva-Font, C., Jin, B.-J., Verkman, AS Aquaporin-1 gene deletion reduces breast tumor growth and lung metastasis in tumor-producing MMTV-PyVT mice.