: The international meeting Light, Endocrine Systems and Cancer--Facts and Research Perspectives was convened because recent research suggests that ubiquitous light may have more serious cancer consequences than expected. Beyond the established causal link between (over) exposure to sunlight and skin cancer, many scientists consider a causal relationship between light, endocrine systems and internal cancers as biologically plausible. To identify options to turn biological plausibility of mechanisms into true understanding and to assess the possible public health relevance we chose to bring together leading specialists from clinical, experimental laboratory and epidemiological studies of these issues to stimulate a critical, multi-disciplinary discussion of published and new results. Presentations at the symposium covered the physics of light and evolutionary aspects and provided intriguing information about chronobiology, physiology and patho-physiology of endocrine systems and carcinogenesis. Experimental and epidemiological findings on light and skin cancer, and of very recent investigations of relationships between light and internal cancers such as breast cancer were presented. The meeting concluded with a lively discussion of future research options. The symposium's essence and constructive atmosphere are captured in these proceedings [Neuroendocrinol Lett 2002 Jul;23 Suppl 2:1-104] which contain: (i) original presentation papers [Vladimir Anisimov, David Blask, Roland Böni, George Brainard, Thomas Erren, Alexander Lerchl, Sidney Perkowitz, Chris Portier, Russel Reiter, Richard Stevens, Günter Vollmer]; (ii) abstracts of oral and poster presentations; (iii) four commentaries [Charles Poole, Chris Portier, Till Roenneberg and Rob Lucas, Vladimir Anisimov and Johnni Hansen] on the presentations at the meeting and possible implications for research and public health. In our view, the biological plausibility of mechanistic links between light and hormones and cancer can serve as a unique basis for syn-disciplinary research and we expect that investigations in this area will become a higher priority research focus.