Horner's syndrome is a complication of epidural analgesia and anesthesia, encountered more commonly in pregnant women than in other patients. Previous reports described the appearance of Horner's syndrome following epidural injection of concentrated local anesthetic solutions. We report unilateral Horner's syndrome occurring in the setting of lumbar epidural analgesia for labor with a very low local anesthetic concentration (bupivacaine 0.04%) in an epidural infusion. We discuss the possible factors that could have contributed to this occurrence despite the extremely dilute concentration of local anesthetic used for analgesia.