Data collected through a National Electronic Injury Surveillance System occupational supplement (NEISS-Work) provide information on persons treated for nonfatal work-related injuries and illnesses in U.S. hospital emergency departments (EDs). CDC's National Institute for Occupational Safety and Health uses these data to monitor injury trends and aid prevention activities. This report summarizes 2004 NEISS-Work injury and illness surveillance data. In 2004, an estimated 3.4 million nonfatal ED-treated injuries and illnesses occurred among workers of all ages, with a rate of 2.5 cases per 100 full-time equivalent (FTE) workers aged > or =15 years. Workers aged <25 years had the highest injury/illness rates. More than three fourths of all nonfatal workplace injuries/illnesses were attributed to contact with objects or equipment (e.g., being struck by a falling tool or caught in machinery), bodily reaction or exertion (e.g., a sprain or strain), and falls. No substantial reduction was observed in the overall number and rate of ED-treated occupational injuries/illnesses during 1996-2004. To reduce occupational injuries/illnesses, interventions should continue to target workers at highest risk and reduce exposure to those workplace hazards with the greatest potential for causing severe injury or death. More emphasis should be placed on prevention-effectiveness studies and dissemination of successful interventions to reduce work-related injuries and illnesses.