In a group of 36 untreated patients with mild to moderate essential hypertension (office systolic and diastolic blood pressures (BPs) 160 ± 3.4 and 102 ± 1.5 mm Hg, respectively), a 24-hour ambulatory BP monitoring and determination of left ventricular (LV) mass index according to the formula of Devereux were performed. After an overnight fast, blood samples were taken for the determination of serum aldosterone, plasma renin activity and serum parathyroid hormone. Urinary catecholamines were sampled for 24 hours. LV mass index (143.7 ± 8 g/m2) did not correlate significantly either with office systolic or diastolic BP. The correlation of LV mass index with mean 24-hour systolic BP (145 ± 3 mm Hg) was statistically significant: r = 0.395, p = 0.026. However, the best correlation was obtained with mean 24-hour diastolic BP (90 ± 3 mm Hg) with r = 0.500 (p = 0.004). Urinary catecholamines were not correlated with LV mass index. LV mass index correlated significantly with plasma renin activity (r = 0.346, p = 0.050), and aldosterone (r = 0.559, p = 0.001). There was a very significant correlation between LV mass index and parathyroid hormone (r = 0.719, p = 0.00001) even after adjustment for mean 24-hour systolic and diastolic BPs. These results clearly demonstrate that ambulatory BP determinants but not office BP parameters are well correlated with LV hypertrophy in essential hypertension. Nonhemodynamic factors are important determinants of LV mass as well. Besides the renin-angiotensin-aldosterone system, parathyroid hormone appears to play an important role in cardiac hypertrophy.