The Japan Nurses' Health Study (JNHS) is a prospective occupational cohort study investigating the effects of lifestyle and healthcare practices on women's health. It was initiated in 2001, with a six-year entry period and a proposed ten-year follow-up. Participants comprise female registered nurses, licensed practical nurses, public health nurses, and midwives, aged 25 yr or over at the baseline survey. Participants were recruited in cooperation with the Japanese Nursing Association and the Japan Menopause Society. A self-administered baseline questionnaire was distributed, requesting demographic information, lifestyle factors, pharmaceutical drug use, physical condition, reproductive health, and disease history. A total of 49,914 women from all 47 prefectures in Japan responded to the baseline survey. Among them, approximately 18,000 agreed to be followed-up, and returned signed informed-consent sheets, together with their completed baseline questionnaires. Changes in lifestyle, healthcare, incidence of disease, and health outcomes over time will now be studied. The cohort receives annual JNHS newsletters and biennial follow-up questionnaires by mail.