Increasing evidence points to involvement of the fibrotic microenvironment in cancer initiation and progression.Here we demonstrate that mice with lung fibrosis challenged with tumors show an increased incidence of lung metastasis and shorter life spans.A fibrotic environment in the lung results in increased abundance of TGFβ31 and CD4+CD25 +FoxP3 + regulatory T cells and a decreased proportion of activated effector T cells.This grave immunosuppressive environment favors tumor localization and growth.