Familysearch.org is an excellent source for World War I service listings. It provides draft records for the years 1917 – 1918, the years the United States fought in this war.
Between April 1917 and November 1918, over 4.7 million American men and women from U. S. Regular forces, National Guard units and draft units fought in the Great War. Over 116,000 were killed or died of disease and another 205,000 were injured. FamilySearch.org provides a state by state listing of those who served their country and has detailed records of draft registration cards and draft notices for World Wars I and II. The site also provides a concise timeline of important events along with other interesting information and links to related sites.