Fish oil and vitamin D have been touted as inexpensive dietary supplements for helping prevent heart disease, stroke, and some kinds of cancer. VITAL — the Vitamin D and Omega-3 Trial — aims to provide hard evidence about whether they actually work, as well as about the safety of high doses of these widely used supplements. The trial, sponsored by the National Institutes of Health, plans to include 20,000 participants — women over age 60 and men over age 65 — representing every state who have never had a stroke or been diagnosed with heart disease or cancer. Participants will be mailed calendar packets containing the study pills, either 2,000 international units (IU) of vitamin D (current recommendations call for 400 IU to 600 IU), or 1,000 milligrams of fish oil, or placebos. The volunteers will not have to see a study doctor or travel to Harvard-affiliated Brigham and Women's Hospital in Boston, which is conducting the study. For more information about VITAL, call (toll-free) 800-388-3963 or visit www.vitalstudy.org.