Concomitant administration of meropenem has been reported to decrease serum level of valproic acid both in humans and in animals. This report describes three children who required simultaneous administration of valproic acid and meropenem. Meropenem rapidly decreased serum valproic acid concentration to sub-therapeutic levels in all three children, and serum valproic acid levels were returned to therapeutic levels in a short time after discontinuing simultaneous meropenem therapy. Valproic acid was not changed to another antiepileptic agent, because no seizure activity was observed. To our knowledge, this is the first case report on the simultaneous administration of meropenem and valproic acid in childhood. In conclusion, it is clear that concomitant meropenem administration decreases serum valproic acid concentration, and we believe that there may be no need to change the antiepileptic drug during this period, provided that the patient has no seizure. More detailed studies are required. (C) 2004 by Elsevier Inc. All rights reserved.