Bioelectrical Impedance Analysis (BIA) is a simple, non-invasive, real-time diagnostic technique routinely used in human clinical practice to assess body composition and hydration status. BIA raw parameters, particularly the Phase Angle (PhA), are gaining attention as markers of congestive states in humans, as body electrical impedance can be affected by variations in body water content. This study aimed to assess whether changes in BIA raw parameters in dogs with myxomatous mitral valve disease (MMVD) could be linked to disease progression. Nine Cavalier King Charles Spaniels (CKCS) with MMVD were prospectively recruited in a longitudinal study design. During the clinical follow-up of each dog, BIA raw parameters - Impedance (Z), Resistance (R), Reactance (Xc), and PhA - were recorded alongside NT-proBNP assessments at two time points. A reduction in PhA recorded at 50 kHz (P = 0.03) and an increase in NT-proBNP concentrations (P = 0.02) were observed in parallel with the progression of MMVD in this group of CKCS between the two time points. These preliminary findings suggest the potential clinical application of BIA in dogs, emphasizing the need for further research to determine whether PhA could serve as a valuable diagnostic and prognostic marker in cardiac diseases, as has already been established in human patients.