Background Whether spinocerebellar ataxia 27B (SCA27B) may present as a cerebellar multiple system atrophy (MSA-C) mimic remains undetermined. Objectives To assess the prevalence of FGF14 (GAA) ≥250 expansions in patients with MSA-C, to compare SCA27B and MSA-C clinical presentation and natural history. Methods FGF14 expansion screening combined with longitudinal deep-phenotyping in a prospective cohort of 195 patients with sporadic late-onset cerebellar ataxia. Results After a mean disease duration of 6.4 years, 111 patients were not meeting criteria for MSA-C while 24 and 60 patients had a final diagnosis of possible and probable MSA-C, respectively. 16 patients carried an FGF14 (GAA) ≥250 expansion in the group not meeting MSA-C criteria (14.4%), 3 patients in the possible MSA-C group (12.5%), but none among probable MSA-C cases. SCA27B patients were evolving more slowly than probable MSA-C patients. Conclusions FGF14 (GAA) ≥250 expansion may account for MSA look-alike cases and should be screened among slow progressors.