The protein selectivity index was measured in 68 patients (53 males, 15 females) with proteinuria due to IgA nephropathy to determine whether it bore any relationship to other clinical and pathological features of known prognostic significance. The mean age of the patients was 25 +/- 8 years with a follow-up period of 42 +/- 35 months. Forty-six presented with asymptomatic haematuria and proteinuria, 17 with macroscopic haematuria and 5 with the nephrotic syndrome. Twenty-three (34%) patients had selective proteinuria and 45 (66%) had non-selective proteinuria. Patients with non-selective proteinuria had more glomerulosclerosis (29% +/- 20 vs. 16% +/- 20, p less than 0.02), higher serum creatinine (1.47 mg/dl +/- 0.70 vs. 1.17 mg/dl +/- 0.33, p less than 0.02), lower creatinine clearance (79 ml/min +/- 28 vs. 95 ml/min +/- 25, p less than 0.02), and higher incidence of hypertension (chi 2 = 3.84, p less than 0.05) when compared to those with selective proteinuria. The protein selectivity was measured at the end of the study. Of the 5 patients with the nephrotic syndrome, 1 had poorly selective proteinuria and failed to remit and 4 had highly selective proteinuria who either remitted spontaneously (1 patient) or with treatment (3 patients). The results suggest that patients with IgA nephropathy and poorly selective proteinuria are more likely to have other features indicating a poor prognosis such as glomerulosclerosis, renal impairment and hypertension.