The first dedicated experiment for the rare kaon decay K{sub L}{sup 0}{yields}{pi}{sup 0}{nu}{nu} has been performed by the E391a Collaboration at the KEK 12-GeV proton synchrotron. A new upper limit of 2.1x10{sup -7} at the 90% confidence level was set for the branching ratio of the decay K{sub L}{sup 0}{yields}{pi}{sup 0}{nu}{nu} using about 10% of the data collected during the first period of data taking.
