The influence was investigated of ceria addition and the grain size on ageing and wear behaviour of Y-TZP ceramics. Two types of ceramics were studied: ZY5 (95 mole% ZrO2 and 5 mole% YO1·5) and ZY4Ce4 (92 mole% ZrO2, 4 mole% YO1·5, and 4 mole%CeO2). Each ceramic was sintered with grain sizes of 0·18 μm (95% dense) or 0·5 μm (98% dense). Only the ZY5 ceramic with a grain size of 0·5 μm showed degradation to monoclinic zirconia (= ageing) after a treatment in water at 180°C. This ceramic was also the only one in which a phase transformation was observed after wear tests in water for a pin (SiC) on plate test configuration. However, this transformation was found not to affect the wear rate. Wear mechanisms were investigated for testing in dry nitrogen and water environment.