Retroperitoneal fibrosis (RF) is a rare condition in which hard scar tissue develops behind the abdomen. It develops because of cancer, inflammation, or for no apparent reason. It first causes symptoms of dull low back or abdominal pain. As it progresses, the scar tissue may press on the kidneys and blood vessels and cause bloody urine, leg pain, and blood clots. Treatment of RF is surgical removal of the tissue. However, because the tissue is extremely hard, it may not easily separate from organs and vessels. As a result, complete surgical removal is often impossible. For this reason, there is great interest in finding drugs to help treat RF. Tamoxifen is a drug that blocks the effects of the hormone estrogen. Reports suggest that tamoxifen is useful for treating RF. This information, however, comes from individual patients treated and monitored differently by different doctors. To be more sure that tamoxifen is safe and effective in treating RF, it would be helpful to have information from a larger group of patients who were treated and monitored similarly.