Background: The present study aimed to identify the frequency of Fabry disease in patients with cardiac hypertrophy of unknown etiology and to evaluate demographic and clinical characteristics, enzyme activity levels, and genetic mutations at the time of diagnosis. Methods: This national, multicenter, cross-sectional, single-arm, observational regis- try study was conducted in adult patients with a clinical echocardiographic diagnosis of left ventricular hypertrophy and/or the presence of prominent papillary muscle. In both genders, genetic analysis was performed by DNA Sanger sequence analysis. Results: A total of 406 patients with left ventricular hypertrophy of unknown origin were included. Of the patients, 19.5% had decreased enzyme activity (≤2.5 nmol/mL/h). Although genetic analysis revealed GLA (galactosidase alpha) gene mutation in only 2 patients (0.5%), these patients were considered to have probable but not “definite Fabry disease” due to normal lyso Gb3 levels and gene mutations categorized as variants of unknown significance. Conclusion: The prevalence of Fabry disease varies according to the characteristics of the population screened and the definition of the disease used in these trials. From cardiology perspective, left ventricular hypertrophy is the major reason to consider screening for Fabry disease. Enzyme testing, genetic analysis, substrate analysis, histopathological examination, and family screening should be performed, when necessary, for a definite diagnosis of Fabry disease. The results of this study underline the importance of the com- prehensive use of these diagnostic tools to reach a definite diagnosis. The diagnosis and management of Fabry disease should not be based solely on the results of the screening tests.