Lanthanum ion (La3+) has been reported to affect proliferation or apoptosis of different cells. In the present study, La3+ was confirmed to promote both proliferation and apoptosis of NIH 3T3 cells at the same concentrations. La3+ was shown to promote proliferation by helping the cells to pass through the G1/S restriction point and enter S phase, however, the proliferating cells induced by incubation with La3+ eventually underwent apoptosis. The proliferation and apoptosis of NIH 3T3 cells induced by La3+ were well correlated with cell-cycle alterations. La3+ caused the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2; while inhibition of ERK phosphorylation by 2'-amino-3'-methoxyflavone (PD98059) suppressed both proliferation and apoptosis induced by La3+. Based on the above experimental results, we postulated that La3+-promoted proliferation of NIH 3T3 cells could be interconnected with the cell apoptosis, possibly through cell-cycle machinery. Our results thus support the recent hypothesis that proliferation and apoptosis of cell are intrinsically coordinated.