Superplastic forming under biaxial tension of tetragonal zirconia (Y-TZP) is investigated by pushing a hemispherical punch (radius 6 mm) on Y-TZP which was placed on a ring with an inner diameter of 16·7 mm. Dense Y-TZP samples with a grain diameter of 125 nm could be elongated to a dome height of at least 8 mm at a temperature as low as 1160°C. Such elongations could not be achieved at this very low temperature when the ceramic had a grain diameter of 250 nm. At a grain size of 250 nm differences in deformation behaviour were observed for different types of Y-TZP powders. This is explained by the fact that at higher cavity concentration in the sintered compact the sample deforms at lower forces but fractures at lower elongations.