Background. Echocardiography provides a noninvasive means of assessing left ventricular (LV) structure and evidence of LV wall remodeling in hypertensive persons. The relation of demographic, biological, and other factors with LV structure can be assessed. Methods and Results. LV structure was assessed by M-mode echocardiograms for 511 men and 333 women with mild hypertension (average blood pressure, 140/91 mm Hg). Measurements of LV wall thicknesses and internal dimensions were made, and estimates of LV mass indexes and other derivations of structure were calculated. LV hypertrophy criteria were based on previously reported echocardiographic population studies of normal subjects. These measures were compared by age, sex, race, body mass index, systolic blood pressure, antihypertensive drug use, physical activity, alcohol intake, cigarette smoking, and urinary sodium excretion. Despite virtual absence of ECG-determined LV hypertrophy, 13% of men and 20% of women had echocardiographically determined LV hypertrophy indexed by body surface area (g/m2) and 24% of men and 45% of women had LV hypertrophy indexed by height (g/m). Black participants had slightly higher mean levels of wall thickness than nonblack participants but similar LV mass. Systolic blood pressure and urinary sodium excretion were significantly and independently associated with LV mass index and LV hypertrophy using both g/m2 and g/m. Body mass index was significantly related to LV mass index and LV hypertrophy using g/m. Smoking was significantly associated with LV mass index, i.e., using continuous measurement but not using the dichotomy for LV hypertrophy. Conclusions. This study of a large population of men and women with mild primary hypertension, largely without ECG evidence of LV hypertrophy, showed a substantial percentage of participants with echocardiographically determined LV hypertrophy. LV mass indexes correlated positively with systolic blood pressure, body mass index, urinary sodium excretion, and smoking.