Various immunological parameters (IgA, IgG, IgM serum levels, C3 and C4 complement components and T and B lymphocytes and T-lymphocyte subsets) were determined in 50 patients with epilepsy (20 without medication, 15 on carbamazepine and 15 on valproate) and in 20 controls. The epileptic group had fewer CD4 and more CD8 lymphocytes, while the CD4/CD8 ratio was lower; the levels of IgG and IgM were increased and C4 complement component was also decreased. Patients without medication showed all the above mentioned abnormalities. Patients on carbamazepine had a lower CD4/CD8 ratio, increased IgG and IgM levels and decreased C4 complement component while patients on valproate had more CD8 lymphocytes, a lower CD4/CD8 ratio, increased levels of IgM and decreased C4 complement component. Our results indicate that there is a defective immune mechanism in epileptic patients modified by treatment.