Lake Kasumigaura and Kitaura in Ibaraki Prefecture is the second largest lake in Japan. In the lake, common fisheries such as trawling and set net as well as net cage culture are practiced. Net cage culture produces about 5,000 tons of edible common carp (Cyprinus carpio) per year, which is one half of the total production in Japan. At the beginning of October 2003, deaths of unknown origin of common carp in net cages occurred in Lake Kasumigaura. After inquiry of the cause, PCR testing detected DNA of the Koi herpesvirus (KHV) in the affected fish and natural carp captured in Lake Kitaura, confirming the occurrence of the Koi herpesvirus disease (KHVD). We conducted an interview survey on aquaculturers regarding the date, quantity, and circumstance of death of cultured carp in Lake Kasumigaura and Kitaura, and incestigated the circumstances of death in net cage farms. Another interview survey was also conducted on other fishermen regarding the occurrence of abnormality in natural carp, and both cultured and natural carps were sampled for PCR testing. The death of cultured carp in Lake Kasumigaura was generated in two areas dense with net cage farms, and then was likely expanded. The cumulative amount of death according to the interview surveys increased rapidly: 200-300 tons in the middle of October, 660 tons at the end of the same month, and 1,200 tons at the beginning of November. The damage amount reached one quarter that of the annual common carp products. Since the KHVD is one of the designated diseases under the Law to Ensure Sustainable Aquaculture Production, to prevent the spread of this disease according to this law, Ibaraki Prefecture requested aquaculturists of Lake Kasumigaura and Kitaura to exercise restraint on the transfer of cultured carp in net cages on November 2, and issued orders prohibiting transfer on November 12 and implementing incineration and landfill disposal on December 21. The incineration disposal commenced from January 20. This report aims to shed light on the state of mass cultured carp kills in Lake Kasumigaura and Kitaura, caused by this KHV, and the state of outbreaks of abnormal fish among cultured and natural carp.