The purpose of this study was to assess clinical variables which might be predictive of echocardiographic left ventricular hypertrophy in mildly hypertensive patients. Eighteen patients (mean age 51) were studied following four weeks of hydrochlorothiazide monotherapy. Variables assessed included age, duration of hypertension, body surface area, serum cholesterol, alcohol consumption, smoking, maximum systolic and mean blood pressures, and variability of blood pressure determined from hourly measurements taken 12 hours after hydrochlorothiazide dosing. Using stepwise multiple linear regression (with left ventricular mass index analyzed as a continuous variable), the variability of blood pressure was predictive of an elevated left ventricular mass index (p less than 0.0003, r2 = 0.61). The duration of hypertension added significantly to the variability in predicting an elevated left ventricular mass index (p less than 0.004, multiple r = 0.74). In conclusion, echocardiographic left ventricular hypertrophy was significantly related to the variability of blood pressure recorded hourly for 12 h after subjects received 50 mg of hydrochlorothiazide.