This article reexamines the conventional wisdom that characterizes Sino-Japanese energy relations as predominantly competitive, but views Sino-Japanese environmental relations as essentially cooperative. Using sociological theories of risk, it is argued that Sino-Japanese cooperation is more likely in both the energy and environmental areas when common risks are perceived and relative gains are minimized. Despite their many conflicting strategic, political, and economic interests, as energy importers who are vulnerable to supply interruptions in the Middle East and competitors for global energy supplies, China and Japan share common risks to their energy security. Consequently, there actually may be increasing opportunities for China and Japan to address their common concerns through bilateral and multilateral cooperative practices, such as common positions on pricing or energy conservation initiatives. Although one would expect China and Japan to highlight their mutual interests in tackling environmental problems such as air pollution, in fact relative gains often impede cooperation. Japan increasingly views China as an economic competitor and is reducing environmental aid, while China continues to set a priority on economic growth, which sets limits on the use of costlier Japanese green technologies. By examining a selection of scholarly articles, reports and newspaper articles by Chinese and Japanese analysts, as well as material from interviews in Beijing and Tokyo in May-June 2007, the paper shows how environmental and energy issues in Sino-Japanese relations may be framed as threats, requiring counter-measures, or common risks, which can be addressed through cooperative practices. Lastly, the paper discusses the possibility of the development of an energy security 'risk community' as cooperative practices develop between China and Japan. Nonetheless, conflicting political interests, strategies, and self-images, accentuating relative gains, may provide obstacles to their cooperation in both energy security and environmental protection.