Chronic inflammatory demyelinating polyneuropathy (CIDP), which can occur through life from childhood to old age, presents a wide variety of clinical phenotypes. We investigated the relationship between age of onset and phenotype in 124 CIDP patients. Clinical symptoms, pathologic findings and electrophysiologic features were assessed according to age at onset: juvenile, younger than 20-years-old; adult, 20 to 64; and elderly, older than 64 (total n=124). Half of the juvenile group showed subacute progression initially, while most patients in the elderly group showed chronic insidious progression (chi(2)=23.2, P<0.0001). Motor dominant neuropathy was prominent in juveniles, while sensorimotor neuropathy was frequent in the elderly group (chi(2)=27.0, P<0.0001). A relapsing and remitting course predominated in the juvenile group (chi(2)=8.50, P=0.0143). Demyelinating and axonal degenerating features in sural nerve biopsy and in nerve conduction studies were common to three age groups. The subperineurial edema was more prominent in the juvenile and adult groups (P=0.006). Functional recovery was common in all three age groups, but was least apparent in the elderly group (P=0.00062). Demyelinating features in studies of nerve conduction and biopsy specimens was common to all three age groups, and was a useful diagnostic feature. Clinical features of CIDP differ by age of onset, which is a factor to consider in diagnosis, therapy, and prognosis.