During treatment with fingolimod, B cells are redistributed from blood to secondary lymphoid organs, where they are protected from the effect of anti-CD20 and other cell-depleting therapies. We describe a multiple sclerosis patient who had almost complete depletion of B cells in blood during and shortly after treatment with fingolimod. He developed severe disease activity resembling immune reconstitution syndrome after switching from fingolimod to rituximab, with first dose being six weeks after fingolimod cessation. Following recommendations from the Swedish MS Association, rituximab treatment was started as one single dose of 1000 mg. In patients treated with fingolimod, pathogenic B cells may still be sequestered in secondary lymph nodes if this dose is given early. To deplete such B cells as they egress from the lymph nodes, we propose that a second dose of rituximab a few weeks after the first dose should be considered.