There is now evidence that chronic fatigue syndrome (CFS) is accompanied by immune disorders and by increased oxidative stress. The present study has been designed in order to examine the serum concentrations of IgA and IgM to LPS of gram-negative enterobacteria, i.e. Hafnia alvei; Pseudomonas aeruginosa, Morganella morganii, Proteus mirabilis, Pseudomonas putida, Citrobacter koseri, and Klebsiella pneumoniae in CFS patients, patients with partial CFS and normal controls. We found that the prevalences and median values for serum IgA against the LPS of enterobacteria are significantly greater in patients with CFS than in normal volunteers and patients with partial CFS. Serum IgA levels were significantly correlated to the severity of illness, as measured by the FibroFatigue scale and to symptoms, such as irritable bowel, muscular tension, fatigue, concentration difficulties, and failing memory. The results show that enterobacteria are involved in the etiology of CFS and that an increased gut-intestinal permeability has caused an immune response to the LPS of gram-negative enterobacteria. It is suggested that all patients with CFS should be checked by means of the IgA panel used in the present study and accordingly should be treated for increased gut permeability.