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BackgroundOnychomycosis is the most frequent nail disorder, but unfortunately, curative treatment is still a challenge, and commonly the infection recurs. A widely disseminated system to accurately assess and classify the severity of this disease, such as the MASI score for melasma or PASI for psoriasis, is lacking in the literature. In 2011, Carney et al. established and successfully validated the Onychomycosis Severity Index (OSI), proving it to be a simple and reproducible tool.ObjectivesTo validate the Onychomycosis Severity Index in a Brazilian population.Materials & MethodsFour experienced dermatologists were taught how to use the OSI system, and then evaluated photographs of 24 nails. There was no consultation between the dermatologists, and the results were evaluated by an impartial third party.ResultsA statistically significant (pConclusionOSI is a very useful tool to improve the clinical assessment of onychomycosis and support clinical trial inclusion criteria (p<0.001). It also provides important prognostic data and allows for a better follow-up of treatment efficacy.