Veterans who have participated in the Gulf War suffer from a number of symptoms, collectively referred to as the Gulf War Syndrome. It has been hypothesized that a change in the systemic cytokine balance or other changes in immunological parameters could be responsible for some of the symptoms. We analyzed the peripheral blood natural killer (NK) cell activity of 686 Gulf War personnel who had been present in the Persian Gulf area during and immediately after the Gulf War as well as 231 gender and age-matched controls. The test material included individual samples of frozen peripheral blood mononuclear cells kept at -139 degrees C for a period of 50 to 380 days prior to NK cell analysis of freshly thawed cells. Significant differences in NK-cell activity were not observed by direct comparison of the levels of natural cytotoxic activity in the two groups. However, NK-cell cytotoxicity as such decreased due to cryopreservation. Surprisingly, the NK cells obtained from control donors were significantly (p