We have evaluated putative predictors of the progression in diabetic nephropathy during long-term antihypertensive treatment. Twenty hypertensive insulin-dependent diabetic patients with nephropathy were followed for three (2 to 5) years before, and for three years during antihypertensive treatment with metoprolol and furosemide. Fall rate in glomerular filtration rate (GFR) was 9.5 +/- 3.8 ml/min/year (mean +/- SD) before and 3.6 +/- 3.6 during antihypertensive treatment. Albuminuria was 1442 (150 to 7564) micrograms/min (median range) in the last year before and 880 (96 to 3310) micrograms/min in the first year during treatment. Relative change in adjusted albumin excretion (ratio of values obtained during first year of treatment/and last year before) was significantly correlated to fall rate in GFR during the three years of treatment (r = 0.46, P < 0.05) and to relative change in fall rate in GFR (fall rate during and before treatment were compared) (r = 0.47, P < 0.05). No significant correlations were found between fall rate in GFR during the three years of treatment and arterial blood pressure, albuminuria or GFR measured the last year before, the first year during treatment or the relative changes in these three variables (after-before). In conclusion, a decrease in fractional albumin excretion during conventional antihypertensive treatment predicts an attenuated fall rate in GFR in diabetic nephropathy. The finding suggests a clinical application in monitoring the efficacy of antihypertensive treatment in diabetic nephropathy.