The side population (SP) in human lung cancer cell lines and tumors is enriched with cancer stem cells. An endogenous inhibitor of angiogenesis known as tissue inhibitor of matrix metalloproteinase-2 (TIMP-2), characterized for its ability to inhibit matrix metalloproteinases (MMPs), has been shown by several laboratories to impede tumor progression through MMP-dependent or -independent mechanisms. We recently reported that forced expression of TIMP-2, as well as the modified form Ala+TIMP-2 (that lacks MMP inhibitory activity) significantly blocks growth of A549 human lung cancer cells in vivo. However, the mechanisms underlying TIMP-2 antitumor effects are not fully characterized. Here, we examine the hypothesis that the TIMP-2 antitumor activity may involve regulation of the SP in human lung cancer cells. Indeed, using Hoechst dye efflux assay and flow cytometry, as well as quantitative reverse transcriptase-PCR analysis, we found that endogenous TIMP-2 mRNA levels showed a significant inverse correlation with SP fraction size in six non-small cell lung cancer cell lines. In A549 cells expressing increased levels of TIMP-2, a significant decrease in SP was observed, and this decrease was associated with lowered gene expression of ABCG2, ABCB1 and AKR1C1. Functional analysis of A549 cells showed that TIMP-2 overexpression increased chemosensitivity to cytotoxic drugs. The SP isolated from TIMP-2-overexpressing A549 cells also demonstrated impaired migratory capacity compared with the SP from empty vector control. More importantly, our data provide strong evidence that these TIMP-2 functions occur independent of MMP inhibition, as A549 cells overexpressing Ala+TIMP-2 exhibited identical behavior to those overexpressing TIMP-2 alone. Our findings provide the first indication that TIMP-2 modulates SP phenotype and function, and suggests that TIMP-2 may act as an endogenous suppressor of the SP in human lung cancer cells.