In a prospective study performed in 1977-1978 (one year), all 90 children (age 6 months to 51/2 years, median 21 months) admitted due to febrile seizures were examined for blood brain barrier function and intrathecal immunoglobulin synthesis. The CSF/serum albumin ratio and CSF total protein reflected the blood brain barrier function, while intrathecal immunoglobulin synthesis was presented by determination of the CSF IgG index [CSF/serum IgG ratio]:[CSF/serum albumin ratio] and the corresponding CSF IgA index. CSF IgM concentrations were also determined. CSF and serum taken at admission were available from 64 children, and CSF and serum taken three to four weeks later from 28. Intrathecal synthesis of one or more of the immunoglobulins was concluded to occur in 42% of the children at admission and in 25% at the follow-up examination. Normal blood brain barrier function and normal serum immunoglobulin concentrations were found. There were no relations to clinical variables.