This study used factor analysis to identify possible subgroups of symptoms of fatiguing syndrome (FS). Subjects were classified with FS according to the 1994 modified Centers for Disease Control and Prevention criteria with the exception that the chronicity of excessive fatigue could not be documented. The cases consisted of 640 Gulf War veterans who met the criteria, whereas the control groups were composed of 5417 Gulf War and 6493 non-Gulf War veterans who did not meet the criteria and had none of the medical conditions that were exclusionary in the FS group. Factor analyses were performed separately in each group. Factor analysis revealed 6 subgroups: fatigue, pain, infectious, gastrointestinal, respiratory, and neurologic/mood/fatigue. Although the factors were similar for both groups, they were more differentiated in the FS group than in the control group as evidenced by interfactor correlations.