Purpose: This study compared the minutes/day of moderate-to-vigorous physical activity (MVPA) and sedentary time (ST) using the vertical axis (uniaxial) vs. vector magnitude (VM: triaxial) counts per minute data in cardiac rehabilitation (CR) patients. Methods: Accelerometry data was collected on 46 (14 women, mean age 62) CR patients. Patients wore an accelerometer for 9 days at the beginning (i.e., within the 1st three weeks), end, and 3-months after completing CR. For the current abstract, data is only available for baseline data collection. Results: Seperate paired sample t-tests were performed to examine whether there were differences in the minutes/day of MVPA and ST calculated using the uniaxial vs. VM data, respectively. Results showed there was a significant difference in both the minutes/day of MVPA [t(43) = -14.12, p = 0.00] and ST [t(43) = 10.46, p = 0.00] using uniaxial vs. VM data. Measurement of agreement between the minutes/day of (a) MVPA for the uniaxial vs. VM, and (b) ST for the uniaxial vs. VM, were exmained using Bland-Altman plots. Results showed that the uniaxial data underestimated participants' daily time in MVPA by 24 (+ 15) minutes and overestimated ST by 72 (+ 34) minutes compared to MVPA and ST calculated using VM data. Conclusion: Utilizing either uniaxial or VM data was found to significantly impact MVPA and sedentary behaviour outcome measures in CR patients. These findings highlight the impact different accelerometer data post-processing procedures can have on outcome measures in this population.