In a study of genetic markers in patients with epilepsy, 30 genetic systems have been tested and the results compared with all previously published studies on this subjects. Only one marker, Ss + ss/SS in the MNSs blood group system showed a statistically highly significant difference (p below 0.001) in the epileptic patients compared with a control group. A previously reported difference in the Pc gene of the red cell acid phosphatase system was not confirmed in the present study. The basis for an association between the S antigen and epilepsy is difficult to understand at present and will need to be confirmed by studies on other groups of epileptic patients and in the aetiologically different groups before being accepted. This is especially so as the Chalfont patients are the only group so far studied for this blood group system.