A recent study reported in the British Medical Journal suggests a link between the concentration of estrogen in drinking water and an increase in prostate cancer in males. This is a first of its kind ecological study. The World Health Organization (starting on page 251 of link) considers estrogen as a Class I carcinogen (like asbestos, radiation and tobacco).
The source of estrogen in the drinking water is a disputed point. Sources of estrogen include oral contraceptives, estrogen replacement therapy and animals.