To test the hypothesis of a difference between men and women in the left ventricular hypertrophic response to diurnal variations of ambulatory blood pressure in essential hypertension. Non-invasive ambulatory blood pressure monitoring and echocardiography in untreated hypertensive patients and healthy normotensive subjects. Community-based ambulatory population in tertiary care centers. Two hundred and sixty hypertensive patients and sixty-three healthy normotensive subjects. Patients with average daytime systolic blood pressure (SBP) and diastolic blood pressure (DBP) falling by less than 10% during the night were defined as non-dippers, the others as dippers. In the hypertensive group, dippers and non-dippers did not differ, in either gender, in several covariates possibly affecting left ventricular structure, including daytime ambulatory blood pressure, prevalence of white coat hypertension, age, body mass index, family history and known duration of hypertension, funduscopic changes, diabetes, alcohol consumption and renal function. Left ventricular mass (LVM) did not differ between dippers and non-dippers in hypertensive men whilst in hypertensive women it was significantly lower in dippers than in non-dippers. This sex difference held for all quartiles of the distribution of mean daytime blood pressure. In hypertensive women there was an inverse correlation between LVM and the per cent reduction of SBP and DBP from day to night, but this relationship was absent in hypertensive men. Other indices of left ventricular structure differed between dippers and non-dippers in both genders, as did LVM. For any level of daytime ambulatory blood pressure, a reduction of SBP and DBP by less than 10% from day to night identifies a subset of hypertensive patients at increased risk of left ventricular hypertrophy only in the female gender. These data suggest that, compared with men, hypertensive women require a longer duration of exposure to high blood pressure levels during the 24 h to develop left ventricular hypertrophy.