To evaluate the effect of blood pressure variability on Riva Rocci Korotkoff blood pressure determinations, we studied the intra-arterial pressure during Riva Rocci Korotkoff determinations in 25 patients. In 50 measurements with a cuff deflation rate of 2.5 mm Hg/sec, the systolic intra-arterial pressure at Korotkoff phase 1 was 2.1 (95% confidence interval 1.2-3.0) mm Hg higher than the average systolic intra-arterial pressure in the 20 seconds surrounding this moment (p less than 0.01). This elevation was shown to increase with increasing blood pressure variability, as expressed by the standard deviation of the systolic blood pressure. The elevation caused by the proposed mechanism was amplified when a slow cuff deflation rate of 1.5 mm Hg/sec was used. No significant difference was noted in the diastolic intra-arterial pressure at Korotkoff phase 5, when a normal deflation rate of 2.5 mm Hg/sec was used. A slow cuff deflation rate caused a small elevation in diastolic pressure at the moment of Korotkoff phase 5. We conclude that even normally occurring blood pressure variability might lead to a consistent elevation of systolic Riva Rocci Korotkoff blood pressure measurements. This effect is reinforced by an increasing blood pressure variability and by a slower cuff deflation rate.