A sample of 143 male borderline hypertensives aged 35 to 45, screened from a population cohort, were subjected to psychological stress, static work, and ambulatory 24 h blood pressure (BP) monitoring to assess the predictive power of BP reactivity in the development of established hypertension. After 1 year, a follow-up showed that 21 subjects (14.7%) had developed established hypertension (causal diastolic (D) BP > or = 95 mm Hg), 25 subjects (15.7%) had become normotensive (causal DBP < 85 mm Hg), and 97 (67.8%) remained within the borderline range (causal DBP 85 to 94 mm Hg). Those who developed established hypertension had considerably higher initial basal resting blood pressures than those who remained borderline: systolic (S) BP 134.2 +/- 12.5 v 127.6 +/- 10.7 mm Hg, P < .05 and DBP 86.8 +/- 7.9 v 80.4 +/- 7.0 mm Hg, P < .01. They had also somewhat higher BP values during mental arithmetic exercises and hand-grip work (peak DBP 101.1 +/- 8.8 v 96.8 +/- 8.7 mm Hg, P < .05, and 131.4 +/- 14.8 v 123.5 +/- 12.9 mm Hg, P < .05, respectively). Those subjects who developed established hypertension had significantly higher 24 h mean blood pressures than those who remained borderline (24 h SBP 133.3 +/- 11.4 v 126.0 +/- 10.1 mm Hg, P < .01, and DBP 84.7 +/- 5.7 v 81.6 +/- 6.8 mm Hg, P < .05). This difference was attributed mainly to the differences found during daytime (07:00 to 19:00) hours but was also found to be nominally dependent upon those found during nighttime (01:00 to 07:00) hours.(ABSTRACT TRUNCATED AT 250 WORDS)