One hundred nine patients with essential hypertension and without either diabetes mellitus or clinical proteinuria were examined to investigate possible racial differences in urinary albumin excretion rates. The black hypertensive patients were found to have significantly higher urinary albumin excretion rates compared with the white patients; in addition, a significantly greater proportion of the black patients than the white patients (32% v 14%) had microalbuminuria, defined as a urinary albumin excretion rate greater than 30 micrograms/min. These differences could not be explained by age, blood pressure, body mass index, glycosylated hemoglobin, serum creatinine, duration of hypertension, or type of hypertension treatment. Hypertensive renal failure occurs six to 18 times more frequently in blacks than in whites; to our knowledge, these data are the first to indicate that microalbuminuria may be more prevalent during the course of hypertension in black patients and thus may be an early marker for end-organ damage susceptibility among hypertensive patients.