Serum immunoglobulin (Ig) levels and various autoantibodies have been determined in 53 epileptics treated with anticonvulsant drugs for more than 10 years and in 53 controls matched for age and sex. Aberrations in both IgG, IgA and IgM values were demonstrated. The mean IgG concentration was significantly lower in epileptics (1217 mg/100 ml) than in controls (1364 mg/100 ml) p less than 0.05). The mean IgA value was 161 mg/100 ml in both patients and controls (F-test, p less than 0.01), but abnormally low IgA values were found in 17%, and abnormally high values in 11% of the epileptics. The mean IgM concentration was 157 mg/100 ml in epileptics and 117 mg/100 ml in controls (F-test, p less than 0.01). Autoantibodies were found significantly more often in epileptics (26.4%) than in controls (3.8%) (2p less than 0.002). The occurrence of autoantibodies could not be related to alterations in the Ig levels. Neither was it possible to correlate the occurrence of mitochondrial antibodies, smooth muscle antibodies and antinuclear antibodies to the slightly abnormal biochemical liver parameters found in these patients. Thus, the abnormalities in Ig levels and the autoimmune phenomena observed in epileptics on long-term anticonvulsant therapy are not intimately related.