A 12-year-old girl was diagnosed as suffering from multiple sclerosis. At age 14, just after recovery from an exacerbation of MS with left-sided optic neuritis, she underwent a Mantoux tuberculin skin test. Within 30 minutes she developed complete irreversible optic neuropathy of the left eye. This case illustrates the urge for caution to perform vaccinations and tuberculin skin tests not only during progressive disease activity of MS, but also in the reconvalescent phase after an acute exacerbation.