1. Both microalbuminuria and left ventricular hypertrophy may reflect target organ damage in essential hypertension. Both are related to the prevailing level of blood pressure and both are associated with an increase in morbidity and mortality. 2. The database of the Hypertension Diagnostic Ser,Sce, a multicentre secondary referral clinic for patients with essential hypertension, was analysed in order to clarify the level of association between microalbuminuria and left ventricular hypertrophy, which might explain the observed increase in morbidity and mortality in patients with microalbuminuria. Microalbuminuria was measured semiquantitatively by urine dip-stix, lifter the exclusion of patients with potential secondary hypertension, renal disease and diabetes mellitus, patients with complete data for microalbuminuria, left ventricular mass (LVM) and 24 h blood pressure monitoring were selected. 3. Data were complete for 703 patients (47% male, age 51+/-12 years) and 42% tested positive for microalbuminuria, Micro-albuminuria was positively related to 24 h systolic blood pressure and weight and was negatively related to age. Left ventricular mass was higher in patients with microalbuminuria (men, 265+/-69 g; women, 207+/-61 g) than in those without (men, 250+/-64 g, P < 0.09; women, 185+/-50 g, P < 0.001). After correction for the effects of gender, body mass index and 24 h systolic blood pressure, the presence of microalbuminuria was associated with an increase in LVM of 10 g (P < 0.05, 95% confidence interval, 2-19 g) 4. Microalbuminuria as measured by urine dip-stix is associated with an increase in LVM in patients with essential hypertension, independent of other known determinants.