Introduction: Based on evidence from animal experiments and case reports, certain occupational exposures may contribute to yet undetected cancer risk in humans. Since traditional epiemiological studies are time consuming, the use of existing information from registers may facilitate the research process. Due to the existence of a unique personal identification number (CPR-number), a national cancer register operating since 1943, a nationwide pension fund scheme with compulsory membership for all employees covering long-term individual employment history, and sometimes with addition of information from external sources, e.g. interviews or job exposure matrices, it is possible to conduct large nationwide studies of occupational exposures and cancer risk in Denmark. Research topics: In this review we give three examples of cancer studies of widespread occupational exposures: formaldehyde, night shift work, and bitumen. CONCLUSION: Results from register-based studies have contributed to the now-existing evidence on occupational exposures and risk of cancer.