The study is a 30 month, prospective, randomized, double-blind clinical trial. All veterans deployed to the Gulf War between August, 1990 and August, 1991 will be eligible. Patients will be considered to have GWI if they have at least two of three symptoms (fatigue, musculoskeletal pain, neurocognitive dysfunction) that began after August, 1990 and that have lasted more than six months up to the present. Four hundred and fifty of these GWI patients from 28 medical centers who test positive for Mycoplasma fermentans, Mycoplasma gentalium and/or Mycoplasma pneumoniae at baseline and have no exclusion criteria will be entered into the study over a one year recruitment period. These mycoplasma positive species patients will be randomized to one of two treatment groups: (1) patients treated with doxycycline for 12 months and (2) patients given placebo for 12 months. Each patient will receive study medication for 12 months and will be followed for an additional six months after the termination of medications. Patients will be seen monthly during the medication phase and at 18 months. Major evaluations will be completed at baseline and at 3, 6, 9, 12 and 18 months. Patients assigned to doxycycline will receive 200mg/day, which they will be instructed to take in the morning.