Treatment of patients suffering from myelodysplastic syndromes and secondary acute myeloid leukemia after MDS is often unsuccessful. Pro-apoptosis with arsenic trioxide has recently been proposed as a novel therapeutic approach. Exisulind is another potentially pro-apoptotic agent, and therefore, we investigated its influence on proliferation, differentiation, cell cycle and apoptosis in two sAML/MDS cell lines, one de-novo AML cell line and healthy CD34+ bone marrow cells. Treatment of sAML/MDS cells with Exisulind clearly inhibited colony formation in the CFU-assays. Interestingly, Exisulind did not alter the percentages of sAML/MDS cells in G1-, G2-, M- or S-phase, but reduced proliferation and induced apoptosis in this cell type. Exisulind had no effect on de-novo AML or normal CD34+ cells. We detected increased c-Jun NH2-terminal kinase activity in sAML/MDS cells treated with Exisulind. Adding a specific JNK-inhibitor to Exisulind-treated sAML/MDS cells partly abrogated apoptosis, thus proving that Exisulind-mediated apoptosis in sAML/MDS cells is dependent on JNK activation. We conclude that JNK is one mediator of apoptosis in sAML/MDS cells treated with Exisulind. Moreover, our data strongly suggests to explore the potential use of Exisulind as a novel, pro-apoptotic therapy for patients with MDS and sAML/MDS.