Chronic inflammatory demyelinating polyneuropathy (CIDP) is characterized by an insidious onset showing progression over two months. However, up to 16% of CIDP patients may show acute presentation similar to Guillain-Barré syndrome (GBS). Such cases are termed acute-onset CIDP (A-CIDP). Distinguishing A-CIDP from GBS, especially the acute inflammatory demyelinating polyneuropathy (AIDP) subtype, is critical because therapeutic strategies and outcomes may differ between the two syndromes. Regarding clinical features, A-CIDP is less likely to have autonomic nervous system involvement, facial weakness, a preceding infectious illness, or the need for mechanical ventilation, in comparison with AIDP. Electrophysiological features are usually quite similar between the two, although follow-up studies may elucidate key differences. Around 8%-16% of GBS patients may show clinical deterioration shortly after improvement or stabilization following initial immunological therapy. Such a situation is termed treatment-related fluctuation (TRF; GBS-TRF). The distinction between GBS-TRF and A-CIDP is an important clinical issue because maintenance treatment is often required in CIDP. The diagnosis of A-CIDP should be considered when the condition of a patient with GBS deteriorates after nine weeks from onset, or when deterioration occurs three times or more.