Arginine deprivation impairs cell proliferation more strong in cancer than in normal cells; thus, it has been proposed that such an effect could be exploited for cancer therapy. We have compared the effect of arginine deprivation on normal and cancer cells, studying growth rate, morphology, and protein expression patterns in immortalized human MCF10a cells and in MCF7 cells. Arginine deprivation forces MCF10a cells into irreversible senescence while the vast majority of MCF7 cells become quiescent and resume normal growth following arginine re-addition. Arginine deprivation induced a significant burst of p21cip1 in both cell lines that were reversible in MCF7 and irreversible in MCF10 cells. In the latter cells, p21cip1 increase was accompanied by a time-dependent increase of p16INK4A. Similar effects could be obtained by treating both cell types with α-difluoro-methyl-ornithine, but not with Nω-hydroxy-L-arginine, drugs that interfere specifically but differently with the major pathways of arginine metabolism. Our data suggest that derangement in polyamine synthesis is the main consequence of arginine starvation.