Records of repeated examinations of blood pressure are increasingly available for primary care patients, but the use of this information in predicting incident hypertension remains unclear, because cohort studies with repeat blood pressure monitoring are rare. We compared the incremental value of using data on blood pressure history to a single measure as in the Framingham hypertension risk score, a validated hypertension risk prediction algorithm. Participants were 4314 London-based civil servants (1297 women) aged 35 to 68 years who were free from prevalent hypertension, diabetes mellitus, and coronary heart disease at baseline examination (the Whitehall II Study). Standard clinical examinations of blood pressure, weight and height, current cigarette smoking, and parental history of hypertension were undertaken on a 5-year basis. A total of 1052 incident (new-onset) cases of hypertension were observed in two 5-year baseline follow-up data cycles. Comparison of the Framingham risk score with a score additionally incorporating 5-year blood pressure history showed, at best, modest improvements in indicators of predictive performance: C statistics (0.796 versus 0.799), predicted:observed ratios (1.04 [95% CI: 0.95 to 1.15] versus 0.98 [95% CI: 0.89 to 1.08]), or Hosmer-Lemeshow chi(2) values (11.5 versus 6.5). The net reclassification improvement with the modified score was 9.3% (95% CI: 4.2% to 14.4%), resulting from a net 17.1% increase in nonhypertensives correctly identified as being at lower risk but a net 7.8% increase in hypertensives incorrectly identified as at lower risk. These data suggest that, despite the net reclassification improvement, the clinical use of adding repeat measures of blood pressure to the Framingham hypertension risk score may be limited.