The effect of simvastatin, a widely used statin for the treatment of hypercholesterolemia, was investigated in the estrogen receptor (ER)-positive MCF-7, and the ER-negative MDA-MB 231 human breast cancer cell lines. Simvastatin induced cell cycle arrest and apoptosis in both cells. These effects of simvastatin were not altered by 17-beta-estradiol treatment. MCF-7 cells express wild-type tumor suppressor protein p53, whereas MDA-MB 231 cells carry a p53 mutation. However, no alteration in the level or localisation of p53 was observed with simvastatin treatment in either cell line. On the other hand, simvastatin strongly stimulated phosphorylation of c-jun which was completely abolished by the c-jun NH2-terminal kinase (JNK) inhibitor SP600125, which also significantly reduced the antiproliferative and apoptotic effects of simvastatin in these cells. In conclusion, we describe here that simvastatin induces apoptosis via involvement of JNK in breast cancer cells independent of their ER or p53 expression status. These findings indicate a great potential for statins for the treatment of cancers resistant to currently used drugs, and target the JNK signalling pathway for a novel approach of breast cancer treatment.