We have previously reported that IFN alpha -chronic treatment for 41 days induced a partial phenotype reversion on HeLa cells along with a down-regulation of HPV18 mRNA levels. However, tumorigenicity of these cells in nude mice was unchanged. Interestingly, after 1 year of IFN alpha -chronic exposition, HeLa cells failed to induce s.c. tumors when injected into nude mice. In such experimental conditions both HPV18 DNA integration pattern and viral DNA copy number present in HeLa cells remained intact in the nontumorigenic phenotype cells. As result of the treatment with IFN alpha, HeLa cells rendered more resistant to lysis mediated by activated natural killer cells in vitro. Furthermore, IFN alpha -chronic treatment was able to induce VEGF and decrease bFGF mRNA expression, suggesting a potential effect on the angiogenic behavior of these tumoral cells, Thus, long-term treatment of HeLa cells with IFN alpha can accomplish a reversion of the malignant phenotype by a sequential multistep mechanism, in which the antiangiogenic effect of IFN alpha could be one of the contributing events.