Introduction. Data regarding the left atrial (LA) electroanatomical substrate in patients with hypertrophic cardiomyopathy (HCM) and atrial fibrillation (AF) are missing. In this electroanatomical mapping (EAM) study, we evaluated the extent of LA fibrosis and its impact on catheter ablation outcomes in patients with HCM and AF. Methods. High-density LA EAM was performed during AF in 28 consecutive patients with obstructive HCM and AF (42.9% displayed paroxysmal AF and 57.1% persistent AF). After propensity matching (PS), 28 non-HCM patients with AF were selected, and served as controls. Two different cut-off values of bipolar signal amplitude were investigated for fibrosis characterization (≤0.25 mV and ≤0.4 mV). HCM patients underwent pulmonary vein antral isolation (PVAI) and roof line, while non-HCM patients PVAI only. Results. After the 3-month blanking period, 10 HCM patients (35.7%) displayed atrial arrhythmia recurrence. Univariate analysis revealed that the extent of LA fibrosis was the only predictor of AF recurrence. HCM patients with arrhythmia recurrence showed significantly greater low voltage areas defined as either bipolar voltage ≤0.25 mV (22.5±10% vs. 5.5±6.4%, p=0.001) or ≤0.4 mV (32±13.9% vs. 5.9±5.1%, p<0.001). The presence of low voltage areas ≤0.4 mV greater than 14.1% of the total LA area also predicted arrhythmia recurrence with excellent sensitivity (100%) and specificity (100%). After PS matching with non-HCM patients, patients with HCM exhibited wider fibrotic regions ≤0.25 mV compared to non-HCM patients (p=0.016). Conclusions. High-density EAM reveals extensive LA fibrotic disease in patients with HCM, an event with certain implications in catheter ablation outcomes.