Since nimesulide, a nonsteroidal anti-inflammatory drug, is known to be a selective inhibitor of cyclooxygenase-2 and shows activity against cancer cells, there has been much interest in developing related molecules with enhanced anticancer properties. Taking in consideration structural features of nimesulide analogues ten new ortho-(akylthio)-N-alkylacetanilides were synthesized and fully characterized. The antiproliferative effect of these acetanilides was evaluated against human breast (MCF-7) and prostate (LNCaP) cancer cell lines as well as normal human dermal fibroblasts (NHDF). In particular, acetoacetanilides with methylcyclohexyl and/or 2,4-dimethylbenzyl groups linked to amide group and/or to sulfur atom had interesting cytotoxicities against human breast cancer cells. Moreover, these groups caused an increase in the antiproliferative effect against both cancer cells. Docking studies revealed the possibility of these acetoacetanilides to be potential ligands of the androgen receptor, though hormone-independent mechanisms may be involved in antiproliferative effects shown by these acetoacetanilides. In addition, 3D-QSAR studies demonstrated that the cytotoxic activity against the human breast cancer cell line was dependent on both bulkiness and electrostatic nature of the N- and S-alkyl groups of acetoacetanilides.