A multicenter, double-blind, placebo-controlled, randomized trial of fish oil in proteinuric patients with IgA nephropathy is being conducted by the Mayo Nephrology Collaborative Group. We completed enrollment of 106 patients into the trial in December 1991. The treatment period is for two years. Hypertension is being managed in all patients with enalapril maleate (Vasotec). We evaluated the associations between a variety of clinical and renal morphologic features and renal function at the entry of all enrolled patients. Among 78 males and 28 females [age(mean +/- SD) 36 +/- 14 years], older age at treatment randomization, hypertension, at disease discovery as well as at study entry, increased fractional excretion of albumin, increased serum triglyceride levels, and more severe tubulointerstitial, vascular, and combined glomerular and tubulointerstitial histologic lesions were all univariately associated (p < or = 0.01) with poorer renal function measured by reciprocal serum creatinine and creatinine clearance levels. In a multiple regression analysis used to predict baseline reciprocal creatinine, the best final model (R2 = 0.48) included male sex (p < .001), hypertension at treatment randomization (p = .001), decreased peripheral blood erythrocytes (p = .001), increased tubulointerstitial score (p = .004), and increased fractional excretion of albumin (p = .025) as independent predictors of decreased kidney function. These associations are similar to those seen in the high-risk subset of patients with IgA nephropathy who develop end-stage renal disease. In the eventual outcome analysis of the clinical trial, we will examine the effects of treatment on the two potentially modifiable risk factors, hypertension and proteinuria, on renal function.