OBJECTIVE To investigate the effects of angiotensin-converting enzyme inhibitor (ACEI) on IgA nephropathy(IgAN) and the factors influencing the therapeutic effects. METHODS 131 cases with IgAN diagnosed by renal biopsy were randomly divided into two groups: ACEI group (benazepril 10 mg/d) and non-ACEI group as control. The pathological changes in the kidney were analyzed using Lee SMK. RESULTS After 1 month treatment of ACEI, the levels of proteinuria and serum cholesterol decreased significantly, meanwhile those of serum albumin and total protein increased obviously in patients with IgAN(P < 0.05). The proteinuria level at the 3rd month after treatment was lower than that at the 1st month, but was same as that at the 18th month. While in the control group the proteinuria level increased and the clearance of creatinine decreased continuously during the observation (P < 0.05). The effects correlated positively with mesangial proliferation and negatively with course of the disease glomerular sclerosis in glomerulus and changes of small arterioles in the kidney. Normal blood pressare normal renal function I approximately II degree of Lee SMK I degee of interstitial changes minimal changes of small arterioles and mild glomerular sclerosis predicted better response to ACEI treatment than hypertension renal insufficiency V degree of Lee SMK IV degee of interstitial changes severe changes of small arterioles and massive glomerular sclerosis. CONCLUSIONS It is suggested that benezepril could definitely reduce the excretion of urinary protein and protect renal function of patients with IgAN and should be used as earlier as possible. Mesangial prolife ration glomerular sclerosis in glomerulus changes of small arterioles in kidney and course of the disease are the major influencing factors of ACEI treatment on IgAN.