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Background: There is anecdotal belief that clinic blood pressure (BP) drops over consecutive measurements. This has led to guideline recommendations to discard the first BP reading, or take only one reading if systolic BP (SBP) <140 mmHg. However, the magnitude and direction of change in SBP over consecutive measurements is not clear, and the effect of age and BP level on this change in SBP is unknown. We investigated these issues, and their effect on hypertension diagnosis. Methods: Duplicate BP (or triplicate if large BP differences) was recorded by oscillometry among 20,752 participants (aged 45[95CI; 45,46] years; males 50%) from the 2011-13 Australian Health Survey. SBP change was defined as the difference between measurements. Results: SBP decreased between the first two measures in only 56%, whereas it increased in 37% and did not change in 7% of the population. There was a strong, age-dependent, J-curved relationship between SBP change and SBP level (p<0.001), with the smallest SBP change corresponding to controlled SBP (100 – 140 mmHg). The age-dependent SBP changes resulted in significant diagnostic reclassification compared with the approach of discarding the first reading; 63% and 35% reclassified from hypertension to normal BP, and 4% and 13% reclassified from normal to hypertension among those aged <50 years and ≥50 years respectively. Conclusions: The assumption that SBP drops over consecutive measurements is false, and significant age-and BP-dependent reclassification of hypertension diagnosis will result if the first SBP is discarded. These findings highlight the need for change to some international hypertension guidelines.