Objective: Cuff-based 24-hour ambulatory blood pressure (BP) measurement is considered as the gold standard for hypertension diagnosis and treatment monitoring, its clinical value being ubiquitously accepted. This study wants to quantify the measurement discrepancies in simultaneous and synchronized 24-hour ambulatory BP measurements on both arms. Identifying and understanding measurement uncertainties could improve future hypertension management and further stimulate research on alternative, cuff-less systems with adequate measurement precision. Methods: We conducted simultaneous 24-hour ambulatory BP measurements on both arms in 52 healthy young adults. Broadly used and validated cuff-devices (BoSo TM-2430) were synchronized to start BP measurement within one second of each other. Further, we recorded cuff-pressures of both cuff-based devices and the hydrostatic pressure differences between both cuffs throughout the recording. We then scored pressure curves for disturbances (e.g., pressure spikes during deflation) and were able to correct measurements for hydrostatic differences between the arms. This enabled us to not only quantify the extent of measurement inaccuracy but to describe its origins and possible countermeasures. Design and method: Measured BP levels from left and right arm showed limits of agreement in the Bland Altman Plot of > 33 mmHg for systolic and > 30 mmHg for diastolic values over 24 h. Daytime values deviated more than nocturnal values (systolic: > 35 mmHg vs. > 27 mmHg systolic, and diastolic: > 32 mmHg vs. > 24 mmHg, respectively). Linear regression retrieved R2 = 0.39 (p < 0.001) for systolic and R2 = 0.20 (p < 0.001) for diastolic values. Exclusion of artefactual cuff pressure curves improved limits of agreement to > 23 mmHg for systolic and > 19mmHg for diastolic measurements. Subsequently, it improved correlation coefficients to R2 = 0.54 (p < 0.001) for systolic and R2 = 0.35 (p < 0.001) for diastolic values. Correction of hydrostatic differences between the arms had a negligible effect on limits of agreement and strength of correlation. Results: Comparison of synchronized 24-hour BP measurements obtained on both arms showed large measurement uncertainties with possible implications on clinical grading of hypertension and dipping pattern. Exclusion of artefactual cuff pressure curves, not detected by the conventional software of BP devices, mitigates the effect but cannot eliminate worrying discrepancies. These results strongly support the need to further investigate the clinical implications of BP measurement uncertainty. They should further stimulate research on alternative means of BP measurement which might overcome some of the problems related to cuff-based BP monitoring.