With its long tradition of population registration, Denmark has outstanding possibilities for occupational health research. The municipality registers date back to 1924, the national death and cancer registers to 1943, and unique personal identification numbers were introduced in 1968. For studies on occupational cancer, the cancer register has been linked with census data, pension data, and personnel files from various companies. Suspected associations between occupational exposures and cancer have been studied. For example, women in dry cleaning exposed to tetrachloroethylene had an excess risk of liver cancer (observed = 14; expected = 5.2; standardized incidence ratio (SIR) = 2.7; 95% CI = 1.5–4.5), and oncology nurses handling antineoplastic drugs had an excess risk of leukemia (SIR = 10.7), based on two cases. The linked registers have also been used to systematically search for associations between occupations and cancer risks (eg, female hairdressers). Cancer patterns differ greatly across countries and across main occupational groups within countries. Future efforts should focus not only on traditional approaches to occupational cancer research but also incorporate indirect influences of the work environment (eg, smoking, parity, age at first birth) and labor market participation on cancer risk.