This case–control study demonstrates that bilateral ptosis due to ocular sympathetic dysfunction is a characteristic feature of Guillain–Barre syndrome (GBS) and apraclonidine can be helpful in unmasking this clinical feature. Five patients with GBS and 9 healthy controls were assessed for ocular sympathetic dysfunction through application of topical apraclonidine to 1 eye. Changes resulting from reversal of ptosis or miosis due to apraclonidine were compared with the eye on the other side with no apraclonidine using photographs. Ocular sympathetic dysfunction in the form of mild bilateral ptosis was found in all 5 patients with GBS recruited in this study. Consistent with previous reports, healthy subjects had no significant response to apraclonidine. Although there was evidence of concomitant pupillary dysfunction in the form of bilateral Horner syndrome in 2 of the patients with GBS with more severe GBS phenotype, this study did not have the statistical power to reach conclusions regarding pupillary dysfunction and disease severity in GBS.