The public health consequences of hazardous substance releases have not been characterized adequately. In response, therefore, the Agency for Toxic Substances and Disease Registry implemented an active, state-based surveillance system. Information is collected with respect to the events, chemicals, victims, injuries, and evacuations. Five states reported 1,249 events during 1990 and 1991. Seventy-two percent of the events occurred at fixed facilities, and 28% of the events were transportation related. In 80% of the events, one chemical was released. The most frequently released chemicals were herbicides, acids, volatile organic compounds, and ammonias. In 204 events, 846 persons were injured and 7 died. Employees were injured more frequently than first responders or the general public. The most frequently reported injuries were respiratory irritation and eye irritation. Evacuations occurred in 14% of the events. These results provide information for preparedness planning and training of first responders and employees.