BACKGROUND: Blood pressure increases transiently after a major earthquake, but the characteristics and the mechanism of this increase are unknown. METHODS: The study involved 124 elderly hypertensive out-patients from two clinics near the epicenter of the Hanshin-Awaji earthquake (7.2 on the Richter scale) for whom ambulatory blood pressure monitoring and assessment of end-organ damage had been performed before the earthquake. RESULTS: During the 1 to 2 weeks after the earthquake, while major aftershocks persisted, mean (±SD) systolic blood pressure was 14±16 mm Hg greater and mean diastolic blood pressure was 6±10 mmHg greater, but these values returned to baseline by 3 to 5 weeks after the earthquake. The earthquake-induced increase in blood pressure correlated significantly with the "white coat" effect ([clinic systolic blood pressure minus 24-hour systolic blood pressure] r=0.34, P<0.001), body mass index (r=0.28, P<0.001), and age (r=0.24, P<0.01). The earthquake-induced blood pressure increase was prolonged in patients with microalbuminuria for at least 2 months after the earthquake, whereas it was less pronounced in patients who had been treated with an alpha-blocker and in patients with diabetes mellitus. CONCLUSIONS: These elderly patients with hypertension had a substantial increase in blood pressure after a major earth-quake; the increase was usually transient, except in patients who had microalbuminuria. The correlation with white-coat hypertension suggests that both phenomena are related to sympathetic activation.