Persistent symptoms after treatment of neuroborreliosis (NB) are not uncommon. There is currently no evidence for improvement of symptoms after repeated or prolonged antibiotic treatment. However, clinical observations have indicated that some patients improve during treatment with doxycycline (DOX), but regain the symptoms some time after completed treatment. This may be due to an immunomodulatory effect of the drug. The aim of this randomised, double-blind crossover study was to determine whether DOX has an impact on the persistent symptoms through alterations in the immune response and whether such an effect can influence the clinical outcome.