The cyclin-dependent kinase inhibitor, p21(WAF1), induces cell-cycle arrest and can act as a tumor suppressor. However, increasing evidence indicates that p21(WAF1) can also increase resistance to some anticancer therapies and thus promote tumor growth. The mechanisms explaining this paradox have not been explained. We found that conditioned media from MCF-7 breast cancer cells transfected with a p21(WAF1)-specific small interfering RNA (siRNA) significantly reduced endothelial cell migration, invasion and vascular sprouting. Liquid chromatography/mass spectrometry analysis of the conditioned media revealed that p21(WAF1) knockdown significantly reduced secretion of thioredoxin (Trx), a redox protein known to promote tumor angiogenesis. p21(WAF1) knockdown decreased Trx enzymatic activity in cancer cells, by effects on the expression levels of intracellular thioredoxin-binding protein 2 (TBP2), known to bind and inactivate Trx. Consistent with these findings, media from cancer cells transfected with TBP2 siRNA promoted endothelial cell invasion and blocked the anti-angiogenic effect of p21(WAF1) siRNA. Addition of Trx siRNA blocked the pro-angiogenic effects of TBP2 siRNA. Chromatin immunoprecipitation assays showed p21(WAF1) bound TBP2 gene promoter. Taken together, our data suggests that p21(WAF1) can induce Trx secretion and angiogenesis in cancer cells, by direct transcriptional repression of the TBP2 promoter.