The effect of silicon ion implantation upon the corrosion resistance and structure of the cast Co-Cr-Mo alloy of the Vitalium type, was examined. The silicon fluences were 1.5, 3.0 and 4.5 x 10(17)Si(+) cm(-2). The surface layer of the Vitalium samples implanted with these silicon doses was found to become amorphous. Further annealing of the samples at 200 degrees C resulted in the Cr3Co5Si2 phase being formed, whereas the amorphous layer was preserved. The Vitalium samples submerged in the 0.9% NaCl solution underwent mainly uniform corrosion, irrespective of whether or not they had been implanted with Si+ ions. With increasing doses of implanted silicon and after annealing at 200 degrees C (samples implanted with 1.5 x 10(17)Si(+) cm(-2)), the corrosion resistance increased. The thickness of the oxide layer formed during the anodic polarization depended on the implanted silicon doses.