Compelling evidence indicates the role of sphingosine kinase 1 (SPHK1) deregulation in the processes of carcinogenesis and acquisition of drug resistance, providing the rationale for an effective anti-cancer therapy. However, no highly selective inhibitors of SPHK1 are available for in vitro and in vivo studies, except for the newly discovered 'SK inhibitor' (SKI). The present study showed that, in a panel of myeloid leukaemia cell lines, basal level of SPHK1 correlated with the degree of kinase inhibition by SKI. Exposure to SKI caused variable anti-proliferative, cytotoxic effects in all cell lines. In particular, SKI induced an early, significant inhibition of SPHK1 activity, impaired cell cycle progression and triggered apoptosis in K562 cells. Moreover, SKI acted synergistically with imatinib mesylate (IM) to inhibit cell growth and survival. Finally, the inhibitor affected the clonogenic potential and viability of primary cells from chronic myeloid leukaemia (CML) patients, including one harbouring the IM-insensitive Abl kinase domain mutation T315I. Due to the fact that the phenomenon of resistance to IM remains a major issue in the treatment of patients with CML, the identification of alternative targets and new drugs may be of clinical relevance.