Sodium meta-arsenite (NaAsO 2 ) has been suggested to play a role both in initiation/progression of prostate cancer and as a future antiprostate cancer drug. We have studied the effects of NaAsO 2 on cell proliferation of prostate cancer and noncancer cells, breast cancer cells, and adrenocortical carcinoma cells in vitro. Further, we have investigated the effect of NaAsO 2 on the androgen receptor. We report that NaAsO 2 alters the cell proliferation of prostate cells, in a hormetic manner, by increasing cell proliferation at low concentrations and decreasing the cell proliferation at high concentrations. No activation of the androgen receptor was detected. We conclude that NaAsO 2 is able to increase cell proliferation of prostate cells in vitro at low concentrations, while it decreases cell viability at high concentrations. This novel finding has to be further addressed if NaAsO 2 should be developed into an antiprostate cancer drug.