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.