Congenital myotonic dystrophy (CDM) is associated with markedly expanded CTG repeats in DMPK. The presence of numerous immature fibers with peripheral halo is a characteristic feature of CDM muscles together with hypotrophy of type 1 fibers. Smaller type 1 fibers with no structural abnormality are a definitive criterion of congenital fiber type disproportion (CFTD). Nonetheless, we recently came across a patient who was genetically confirmed as CDM, but had been earlier diagnosed as CFTD when he was an infant. In this study, we performed clinical, pathological, and genetic analyses in infantile patients pathologically diagnosed as CFTD to evaluate CDM patients indistinguishable from CFTD. We examined CTG repeat expansion in DMPK in 28 infantile patients pathologically diagnosed as CFTD. Mutation screening of ACTA1 and TPM3 was performed, and we compared clinical and pathological findings of 20 CDM patients with those of the other cohorts. We identified four (14%) patients with CTG expansion in DMPK. ACTA1 mutation was identified in four (14%), and TPM3 mutation was found in two (7%) patients. Fiber size disproportion was more prominent in patients with ACTA1 or TPM3 mutations as compared to CFTD patients with CTG expansion. A further three patients among 20 CDM patients showed pathological findings similar to CFTD. From our results, CDM should be excluded in CFTD patients.