We measured Na+/Li+ CT in 16 IgA nephropathy patients. Records were reviewed (mean observation period 5.5 years) for serial measurements of blood pressure (BP), urinary protein excretion, GFR (51Cr-EDTA) and plasma creatinine. Na+/Li+ CT correlated with the slope of the plot of GFR versus time (rs = -0.66, p = 0.005) systolic BP at diagnosis (rs = 0.62, p = 0.011) and both systolic and diastolic BP at the end of follow-up (rs = 0.69, p = 0.003, and rs = 0.56, p = 0.023). A diastolic blood pressure (DBP) > or = 95 mm Hg was associated with a faster rate of GFR decline (rate of change of GFR: -0.40 vs. -0.14 ml/min/month, p = 0.07; for DBP > or = 95 vs. < 95 mm Hg, respectively). In a multiple regression analysis with the rate of decline of GFR as dependent variable, Na+/Li+ CT emerged as a significant and independent determinant of the rate of fall of GFR (beta coefficient -1.56, SE beta 0.49, p = 0.006) and explained 52.7% of the variation in the GFR fall. Higher activities of Na+/Li+ CT are significantly associated with an increased rate of deterioration of renal function in IgA nephropathy; part of this effect could be mediated by higher blood pressure values.