The value of microalbuminuria in predicting hypertensive complications in pregnant patients at high risk was tested in a prospective trial. A secondary aim was to compare the urinary albumin excretion rate between high- risk hypertensive pregnant patients (study group) and pregnant patients at high risk of other complications, normal pregnant subjects, and nonpregnant subjects. Over the last 5 years, 276 patients were studied (142 in the study group v 134 controls). Albumin was measured in an 8-hour overnight urine collection throughout pregnancy using a radioimmunoassay technique. The pregnant women in both the study and control groups demonstrated a statistically significant increase in albumin excretion rate in the second and third trimesters compared with the first. Mean albumin excretion rate values were significantly higher in the study group (P = 0.0001). Using logistic and linear regression models, the presence of microalbuminuria in the early third trimester was proven to be predictive of hypertensive complications (odds ratio, 2.1; confidence intervals, 1.26 to 3.53) and birth weight (R2 = 0.7, P < 0.05) in the study group. Intrauterine growth retardation and neonatal outcome were less predictable. With the introduction of radioimmunoassays and in light of these significant clinical results, we believe that high-risk patients in whom abnormal proteinuria develops usually have a microalbuminuric phase weeks earlier, and this test has some predictive value for severe disease. In addition, the accepted definition of gestational proteinuria should be reconsidered.