INTRODUCTION: Temporal lobe epilepsy (TLE) is the most frequent focal epilepsy in adulthood. Catamenial C1-type TLE, is characterized by a cyclic seizure exacerbation during the menstrual phase. The heart rate variability (HRV) analysis assesses cardiac autonomic control and may represent a biomarker for Sudden Unexpected Death in Epilepsy (SUDEP). It is plausible that female sex hormones can influence HRV. These changes might be more pronounced in patients suffering from catamenial C1-type TLE where hormonal changes also increase seizure susceptibility. To that aim, we evaluated HRV changes during the menstrual phase of women suffering from catamenial C1-type TLE. METHODS: We enrolled 12 adults with a diagnosis of catamenial C1-type TLE (Catamenial Group) and 12 age-, and seizure-frequency-matched controls with TLE (Non-Catamenial Group). Each patient underwent a 20-minute EEG鈥?鈥疎KG recording in resting state during the menstrual phase. HRV parameters were calculated with a short-lasting analysis of EKG records. Time domain-related, frequency domain-related, as well as non-linear analysis parameters, were compared between the two groups. RESULT: Compared to the Non-Catamenial Group, the Catamenial Group showed significant reductions in SDNN (p-value鈥?鈥?.01), RMSSD (p-value鈥?鈥?.04), pNN50 (p-value鈥?鈥?.001), LnLF ms(2) (p-value鈥?鈥?.05), LnHF ms(2) (p-value鈥?鈥?.007), SD1 (p-value鈥?鈥?.02), and SD2 (p-value鈥?鈥?.01). These results were independent from age, disease duration, numbers of ASM, and seizure etiology. CONCLUSION: Our data provide experimental evidence that vagal output is reduced during the menstrual phase in patients with catamenial C1-type TLE. These results indicate that, during the menstrual phase, patients with catamenial C1-type TLE may be at a higher risk of developing cardiac dysfunctions and SUDEP.