Abstract
Serum levels of citrulline, a product of nitric oxide synthase activity, were measured in 36 CFS patients and 16 controls to determine whether synthase activity may be elevated in CFS patients. Serum citrulline levels were found to be significantly elevated in CFS patients and, in addition, there was a trend towards higher levels in CFS patients with stronger symptoms. These results provide support for the view that nitric oxide synthase activity tends to be elevated in CFS patients, thus supporting a prediction of the elevated nitric oxide/peroxynitrite theory of CFS etiology.