Cohort Profile: The Norwegian Women and Cancer Study--NOWAC--Kvinner og kreft

In the early 80s, a case-control study in California found an association between use of combined oral contraceptives (OCs) and breast cancer risk1 inducing great public concern in many countries. A group of researchers in Norway and Sweden therefore decided to conduct a population-based case-control study of OCs and breast cancer among young women.2 They found an elevated breast cancer risk for both total duration and for current OC use. However, the findings were strongly debated,3 and in several later review articles possible sources of bias in this study were proposed.4,5 Because of the potential problems of selection bias, recall bias and survivor bias with case-control studies, we decided to establish a prospective cohort study to explore the original hypothesis of use of OCs being a risk factor for breast cancer, as well as explore other risk factors for breast cancer. The data collection for the Norwegian Women and Cancer (NOWAC) cohort study started in 1991. In 2002, the prospective analysis of OC use and breast cancer incidence6 confirmed the original findings from the case-control study.