Approximately 10% of patients with systemic lupus erythematosus (SLE) develop epileptic seizures. When occurring before the onset of generalized SLE, the seizures are mainly primary generalized. Accordingly, long-term treatment with anti-epileptic drugs may precipitate SLE, or epilepsy and SLE may both occur as manifestations of a genetically determined predisposition. Some patients develop IgA deficiency during phenytoin treatment. This condition is reversible and IgA becomes normalized when phenytoin is withdrawn (drug-induced IgA deficiency). Some epileptic patients have a drug-independent IgA deficiency. Patients with drug-induced IgA deficiency are usually HLA-A2, while those with drug-independent IgA deficiency are HLA-A1,B8. The gene coding for IgA deficiency seems to be located in the HLA complex on chromosome 6. The gene locus for juvenile myoclonus epilepsy and related disorders is also on chromosome 6 and in close relation to the gene locus for the HLA system. Juvenile myoclonic epilepsy may be accompanied by drug-induced IgA deficiency, but there are also cases with other sometimes less-defined epilepsies, associated with this anomaly. It is possible that the relationship between epilepsy and immune disturbances is related to a common genetically determined susceptibility.