The anti-leprosy drug Clofazimine is known to inhibit respiratory function and hence energy metabolism in yeast and in transformed fibroblasts. The aim of this study was to examine the effect of Clofazimine on the energy metabolism of a chemoresistant human non-small-cell bronchial-carcinoma cell line (WIL) and to determine whether this agent might inhibit the growth rate of this cell line in vitro and in vivo. Oxidative phosphorylation was estimated in vitro by measuring oxygen consumption polarographically and glycolysis was estimated from lactate production. In cells that had been pre-treated with an ATP synthetase inhibitor (oligomycin), the addition of Clofazimine resulted in an increase in oxygen consumption similar to that observed with 2,4-dinitrophenol, a classical inhibitor of oxidative phosphorylation. This inhibition of mitochondrial function was associated with an increase in lactate production. Cellular ATP levels were maintained, possibly indicating a compensatory increase in ATP production via glycolysis. Clofazimine was shown to have a direct cytotoxic effect in vitro with an ID50 of 10.2 μM-When Clofazimine was administered to athymic mice bearing WIL as a subcutaneous xenograft, tumour growth rate was significantly reduced, so that after 3 weeks, tumour size was one third that of controls (p < 0.01). These results suggest that selective inhibition of tumour energy metabolism with agents such as Clofazimine is a potential novel approach to cancer treatment.