A growing body of evidence suggests that agents that inhibit the angiotensin-converting enzyme are renoprotective. In experimental animal models of chronic renal injury, such renoprotection can virtually eliminate progression of the renal injury, provided that therapy is started at the time of injury. In humans with chronic renal injury, renoprotection has been successfully demonstrated only late in the course of the renal disease. The rate of progression to end-stage renal failure can be delayed, but progression continues at a slower pace. Further study is required to determine whether earlier intervention can better preserve nephron structure and function. A strategy for future trials is recommended. It emphasizes more sensitive outcome measures so as to achieve greater statistical power.