We have used the HPRT mutant clonal assay to determine the frequency of mutant T lymphocytes (FMC), as a measure of recent T cell stimulation, in the blood of patients with Guillain-Barré syndrome (GBS) and chronic inflammatory demyelinating polyneuropathy (CIDP). We found that, compared to healthy controls, the FMC in patients with GBS (16) and CIDP (10) was significantly increased in the progressive phase of the neuropathy. FMC returned to normal values during recovery, suggesting a relationship between FMC and disease activity. No correlation was found between FMC values and motor deficit or severity of the neuropathy. The FMC of the GBS patients with a history of infection before onset of neurological symptoms or with insufficient respiration was not significantly different from the other GBS patients. Immunophenotypic analysis showed that the fraction of CD8+ HPRT mutant T cell clones was significantly increased in GBS patients (48%) compared to healthy controls (3%) or CIDP patients (4.5%). Our results are compatible with the notion that T cells are involved in the pathogenesis of demyelinating inflammatory neuropathies.