Human breast cancer MDA-MB-231 cells in vitro overexpress iNOS after a 5-aminolevulinic acid (ALA)-photodynamic therapy (PDT)-like challenge. iNOS-derived NO acts cytoprotectively and stimulates surviving cell growth and invasion. ALA-PDT activates/upregulates pro-survival/progression effector proteins in vitro. ALA-PDT suppresses SCID mouse-borne MDA-MB-231 tumor xenografts and iNOS inhibition enhances this suppression. ALA-PDT upregulates iNOS protein and NO-derived nitrite in MDA-MB-231 tumors.