T-cell activation and proinflammatory cytokines seem to be important in promoting the disease activity in Guillain-Barré syndrome (GBS). Transforming growth factor-beta 1 (TGF-beta 1) is a multifunctional peptide with potent immunosuppressive activity, and can therefore be considered a putative disease-limiting cytokine. We determined levels of soluble TGF-beta 1 in the serum of 12 patients with GBS in serial investigations during the course of the disease, in 12 patients with other noninflammatory neurological diseases (OND), and in 12 healthy control subjects. Levels of biologically active and total TGF-beta 1 were significantly increased in patients with GBS compared with patients with OND and healthy controls. During the course of GBS, levels of TGF-beta 1 peaked in the plateau phase before onset of recovery. During the recovery phase levels of TGF-beta 1 decreased but still exceeded significantly the levels in patients with OND and healthy controls. The differences were more marked with biologically active than with total TGF-beta 1. The temporal relationship between increased serum levels of TGF-beta 1 and the end of the progressive phase indicates that TGF-beta 1 has a role in terminating the pathological immune response in GBS. These findings suggest that TGF-beta 1 may be important in recovery from GBS.