The purpose of this study was to determine if systolic blood pressure (SBP) by itself is sufficient for the JNC-VI (Sixth Report of the Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure)-based classification of blood pressure of Tehranian adult population. Clinically, SBP and diastolic blood pressure (DBP) are sometimes at different stages in the same individual and the higher stage is considered to classify blood pressure level. The prevalence of disparate levels of SBP and DBP has only recently been noticed. Some researches have reported the importance of SBP level, and not DBP, in determining the appropriate classification of hypertension even in those undergoing treatment. Data were collected for 3823 men and 5159 women aged 20-69 years who were not using antihypertensive medication, in the Tehran Lipid and Glucose Study (TLGS), a cross-sectional phase of a large epidemiological study first established in 1999. The study used the mean of two separate blood pressure measurements in each individual. High blood pressure is defined according to the highest level of SBP or DBP. In 86.3% of the subjects, blood pressure stage was determined according to SBP and in 90.0% of them according to DBP. In 77.4% of the subjects (75.7% of men and 78.7% of women), SBP determined blood pressure in the same stage as DBP did. The role of SBP was the most prominent in age groups 20-29 and 60-69 years (91.4 and 90.8%, respectively) and the least in age group 40-49 years (80.4% of the subjects). DBP had a more prominent role in younger ages and the least significance in older ages. In conclusion, SBP has a more prominent role than DBP in determining blood pressure stage according to JNC-VI only in the 60-69-year-old group. The role of DBP is more prominent in other age groups.