Immunoadsorption plasmapheresis (IAPP) is a method of removing circulating immune factors that is used to treat Guillain-Barré syndrome (GBS). We retrospectively analyzed the data on our GBS patients. In 21 patients treated with IAPP, linear regression analysis showed that the time from the onset of symptoms to the initiation of IAPP was correlated with the time required for improvement by one Hughes functional grade. We investigated the critical day for initiating treatment, which we defined as the day when initiation of IAPP was significantly more likely to improve function by at least one Hughes grade when compared with the outcome in patients receiving supportive therapy (non-IAPP group). The critical day was found to be day 6 after the onset of GBS.