Fourteen of 3,754 U.S. Marines who participated in a joint United States-Republic of Korea training exercise during the autumn of 1986 developed hemorrhagic fever with renal syndrome (HFRS). Clinical and laboratory findings among cases included fever, headache, fatigue, gastrointestinal dysfunction, thrombocytopenia, and proteinuria. Ten individuals were hospitalized; 2 died. No subclinical infections were identified through a post-deployment screen of sera obtained from 2,053 exercise participants. Analysis of questionnaires identified no environmental, occupational, or temporal factors as risks for developing disease. However, 13 of the 14 cases occurred among individuals housed at 1 of the 2 base camps used during the exercise. This outbreak represents the largest cluster of HFRS cases among U.S. personnel in the Republic of Korea since the Korean conflict.