The claim that excessive work can cause death is increasingly recognized in Japan, where over 10,000 victims are believed to die annually from karoshi, death from overworking. This study explores the process in which the issue of karoshi became a major social problem in 1990s and continues to be a source of legal disputes in Japan. Through a year long participant observation of anti -karoshi activism and over 120 in-depth interviews with medical and legal professionals, labor unions, victims' colleagues and families, and government officials, it examines the complex multiple processes in which the concept of karoshi was constructed and mobilized as a resistance against corporate power. Karoshi suddenly became a public issue in the late 1980s, mirroring the nation's self-reflection on what it means to be an affluent society. The "discovery" of karoshi was first made in the 1970s, as a result of medical summary of workers' knowledge, in which a group of pro-labor physicians defined the problem as a "rationalization disease" and coined the term as a warning against intensifying and mechanizing production processes. After simmering in the background for 15 years, a group of progressive legal professionals successfully mobilized wives and mothers of karoshi victims for legal activism, framed the issue as a health concern of the average household, and institutionalized ways of addressing karoshi grievances. The participation of housewife activists, who emphasized their mother and wife-hood in gaining the support of the public, galvanized anti-karoshi activism. Debates surrounding karoshi legal cases are infused with moral rhetoric of duty, responsibility and benevolence in labor relations. Capitalizing on liberal Japanese labor laws, anti-karoshi activists stress the moral and legal responsibilities of employers to protect employees' wellbeing. Despite their success in impacting law and policy, their emphasis on employers' duties ironically results in their inability to address the broader context of overworking and the root causes of the complex issue. Furthermore, their strategic focus on workers' compensation paradoxically results in affirming single-minded devotion to work required in corporate culture and the image of Japanese workers as "corporate warriors" that they set out to oppose in the first place