This publication is about the study protocol of the German Cohort Study on Women's Health. The main objective is to investigate medical benefits of a long-term oral contraceptive use. The design is an analytical cohort study based on inquiries. Additional cases will be recruited to analyse rare events in separate case-control studies. Voluntary participants who signed to participate in a long-term study are included. An annual drop-out rate of 15% is expected. Study variables encompass personal characteristics, lifetime history of diseases, but also disturbances of the state of health, and quality of life. It is anticipated to achieve 400,000 women-years of observation by 2001 (historic and concurrent follow-up). The study started April 1, 1998 and the current financial phase finishes December 31, 2001. 6000 participants were recruited until December 1998 equivalent to about 190,000 observation-years. Until the end of 1999, an additional 70,000 women-years should be included. There have been many suggestions from participants' to include additional issues of women's health into the study.