Although several biological agents have been linked convincingly with increased risks for cancer in humans, only infection of the lower female genital tract with human papillomavirus (HPV) and colonization of the gastrointestinal tract with Helicobacter pylori are of importance for cancer occurrence in the Nordic countries. HPV types 16 and 18, and probably types 31 and 33, are causally linked with cervical cancer and H. pylori, a Gram-negative bacteria, with gastric cancer. The estimated prevalence of genital infections with oncogenic types of HPV among Nordic women and the associated risk estimates indicate that approximately 45% of all cervical cancers arising in the Nordic countries are caused by such infections. This will be equivalent to about 550 cases in these countries each year around 2000, or 1% of all cancers arising in these populations. Similarly, the 35% prevalence of persistent gastric infection with H. pylori in the adult Nordic population and the associated risk estimates indicate that some 58% of all gastric cancers in the Nordic countries are due to this bacteria. This will correspond to about 2,350 new cases annually around the year 2000, with 1,350 among men and 1,000 among women, or approximately 2% of all cancers arising in the Nordic population.