[en] On Thursday morning, 30 September 1999 around 10:35 a.m., a nuclear accident occurred at JCO Co. Ltd. in Tokai, Ibaraki Prefecture, Japan and the criticality continued for about 20 hours until the next morning. Three workers at the JCO were heavily exposed to radiation. In spite of intensive care in the hospitals, two of them passed away in December 1999 and April 2000. Dose assessment and health care checks of the inhabitants around the facility were also carried out by the government. This paper provides the details of the accident and the dose assessments. Copyright (2000) Australasian Radiation Protection Society Inc