We evaluated the presence of proximal renal tubular dysfunction as measured by urinary retinol-binding protein (RBP) in 70 patients with systemic lupus erythematosus. Renal disease activity was assessed using the British Isles Lupus Assessment Group (BILAG) index. This is a clinical-laboratory score based on the principle of the physician's intention to treat. Increased urinary RBP (> 400 micrograms/l) was detected in 17 of 22 (77%) patients with active nephritis, six of 18 (33%) patients with probably active nephritis, one of nine (12%) cases with stable renal disease, and one of 21 (5%) cases without apparent renal disease (P < 0.01). Compared to initial values, mean urinary RBP decreased significantly in six patients evaluated after improvement of the exacerbation of renal disease. There was a positive correlation between urinary RBP and 24-h proteinuria (r = 0.40, P < 0.01), and an inverse correlation between urinary RBP and creatinine clearance (r = -0.60, P < 0.01). In a multivariate analysis adjusting for duration of disease, blood pressure, 24-h proteinuria, and creatinine clearance, mean urinary RBP continued to be significantly and progressively greater for patients with no renal disease, stable renal disease, probably active and active nephritis. Proximal tubular dysfunction is frequent in patients with active lupus nephritis. This association cannot be completely explained by the effects of increased total proteinuria, reduced glomerular filtration rate, and systemic hypertension. Urinary RBP seems to be a marker of renal disease activity. This test may be clinically useful to differentiate patients with active lupus nephritis from those with stable or absent renal disease.