Mixtures of Zr(OH)(4) and ZrO2 particles, similar to 10 mn in size, were hydrothermally treated in 0.25-1.5 mol/L H2SO4 solutions at a temperature of 200 degreesC. After 3 h, very short ZrO2 fibers, 10-30 mu in length, were obtained, with no other zirconium compounds observed. The particles grew with treatment time and resulted in whisker particles. In a higher concentration (3 mol/L) H2SO4 solution, ZrO2 whiskers were not obtained, and clear solutions resulted with the starting ZrO2 particles remaining. It was concluded that Zr(OH)(4) was useful as a starting material and that nanosized ZrO2 particles served as seed crystals for whisker formation.