HBPM guidelines state that morning and evening measurements should be recorded and, to improve stability, the first day of measurements should be discarded. Our objective was to assess the reproducibility and reliability of a 4-day HBPM protocol with and without first day measurements. We analyzed a retrospective cohort of ambulatory patients who required a HBPM for diagnostic purposes or evaluation of treatment efficacy. A 4-day protocol was implemented, with daily duplicate measurements in the morning, afternoon and evening, using an OMRON 705 CP validated equipment. HBPM reproducibility was quantified by test-re-test correlations and standard deviation of differences (SDD) between BP measurements obtained during the entire 4 days, with and without exclusion of the first day. The reliability criterion was the stabilization of the mean and standard deviation (SD). We included 353 subjects with a total of 8224 BP recordings (median of 24 recordings per patient). We found a strong test-re-test correlation between days 1 to 4, which improved when we excluded the first day (p<0.001). We also found a reduction of the mean BP when we increased the number of days and a reduction of SDD when we excluded day 1. Therefore, we conclude that the exclusion of the first day of measurements improves the reproducibility and reliability of a 4-day protocol, and such two factors are not affected by the inclusion of afternoon measurements.