Treatment-related fluctuation (TRF), only defined in Guillain-Barre syndrome (GBS), refer to the deterioration of symptoms following treatment-induced improvement, and implies disease activity lasting beyond the effect of immunotherapy. Here, we first propose the concept of TRF in subacute inflammatory demyelinating polyneuropathy (SIDP) with description of a corresponding case. A 27-year-old female presented with acute flaccid paralysis, and experienced two sequential episodes of TRF, the latter occurring around 8 weeks from disease onset. She eventually recovered through intravenous immunoglobulin treatment, and has not experienced any further deterioration over the next four years. The concept of SIDP-TRF would resolve the gap between GBS-TRF and acute-onset chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and help to decide the optimal treatment strategy in a spectrum of idiopathic immune-mediated polyneuropathies.