• In a survey of 1011 pediatric patients with seizure disorders, 93 children (9.2%) were found to have depressed serum IgA concentrations when compared with age-matched controls; 27 of these values were less than 0.1 g/L (<10 mg/dL). Two thirds (64/93) of these patients were being treated with phenytoin, and ten had been previously treated with phenytoin. No relationship between IgA deficiency and serum phenytoin concentration nor use of other anticonvulsant medications was found. The prevalence of phenytoin-induced IgA depression was similar in patients with "primary" or "secondary" seizure disorders. Approximately 40% of the patients with low serum IgA concentrations had mild to moderate depression of serum IgG and/or IgM concentrations when compared with age-matched controls.