This review critically examines the rise in incidence in non-Hodgkin's Lymphoma (NHL) in selected parts of the world and concludes that much of the increase is likely to be due to unknown biological factors. It postulates that one reasonable explanation for the rise in incidence of NHL is the ever increasing exposure of populations, particularly in the Northern Hemisphere, to sunlight. Support for the hypothesis is supplied from both epidemiological evidence and studies of the responses of lymphocytes to ultra-violet light in both in vitro and in vivo experimental work.