In immunoglobulin A nephropathy (IgAN), the abnormal expression of intercellular adhesion molecule-1 (ICAM-1) on proximal tubule epithelium is associated with the glomerular and interstitial infiltration of leucocytes, but its clinical significance remains uncertain. We analysed the relationship between the ICAM-1 (CD54) expression in tubular epithelial cells and interstitial leucocytes, macrophages (CD14) and T lymphocytes (CD3) with the histologic features, proteinuria and serum creatinine at the time of renal biopsy and after 2.42 years in 45 patients with IgAN and after 1.8+/-1.5 years in 29 patients with non-glomerulonephritis (non-GN). In IgAN, ICAM-1+ tubule epithelium was 0.1+/-0.18 (x+/-SD), and this was associated with extracapillary proliferation (up to 20% of Bowman's space), glomerular sclerosis involving less than 50% of glomerular area, interstitial cellular infiltration, tubular atrophy and proteinuria level. ICAM-1+ interstitial leucocytes were correlated with glomerular sclerosis involving less than 50% of glomerular area, glomerular sclerosis involving more than 50% of glomerular area, tubular atrophy, interstitial fibrosis and serum creatinine level. In patients with an increase of 50% in serum creatinine, ICAM-1+, CD14+ and CD3+, interstitial leucocytes were significantly outnumbered than in patients with stable serum creatinine. In non-GN, ICAM-1+ tubule epithelium was 0.02+/-0.04 (U=344, P<0.05, vs IgAN), and this was inversely correlated with the percentage of the normal glomeruli and associated with glomerular sclerosis covering more than 50% of glomerular area, tubular atrophy and serum creatinine level. The association between tubular ICAM-1 and proteinuria and the association between interstitial ICAM-1+, CD14+ and CD3+, leucocytes and renal failure at presentation and the deterioration in IgAN in contrast with non-GN suggest that tubular and interstitial expression of ICAM-1 may be a marker of tubulointerstitial disturbance in IgAN.