α1-Microglobulin (α1M) is a low molecular weight protein and has been best characterized for detecting acute lesions of proximal tubules (Bonventre in Contrib Nephrol 156:213-219, 2007). This study has tried to evaluate the use of α1M for the differential diagnosis of chronic interstitial nephropathy. 145 patients were recruited [81 men and 64 women, mean age 61.8 ± 16.7 years, 64.8 % have an estimated glomerular filtration (GFR) <60 ml/min]. Urinary α1M was evaluated using an immunonephelometric assay. 82 patients were diagnosed as having chronic interstitial nephritis (CIN), and 46 patients have been previously diagnosed of glomerulonephritis (GN). A group of hypertensive patients without renal disease was used as control (n = 17). Patients in GN group had the highest α1M excretion (15.05 mg/24 h). When the α1M/albuminuria rates were calculated, the CIN group had the highest rate (1.03 mg/mg) and the GN group had the lowest rate (0.04 mg/mg) (p < 0.001). When the α1M/proteinuria rates were calculated, the results were rather similar. The AUC for CIN group was 0.785, and the one for GN group was 0.139. Patients with estimated GFR <60 ml/min showed a higher excretion of α1M (18.75, 8.75-40.00 mg/24 h). Nevertheless, α1M/albuminuria and α1M/proteinuria rates were still higher in CIN patients with GFR ≥60 ml/min. α1M urinary excretion is increased in chronic interstitial nephropathy and glomerulonephritis as well as in patients with GFR <60 ml/min. The α1M/albuminuria rate and the α1M/proteinuria quotient are increased in chronic interstitial nephropathies but decreased in glomerular diseases.