The cell populations infiltrating the kidneys and the LFA-1 expression were studied in renal biopsy specimens from patients with proteinuric (n = 15, group 1) and non-proteinuric (n = 12, group 2) immunoglobulin A nephropathy. Both groups were matched for age and renal function at the time of biopsy. Proliferative glomerular changes were more commonly see in group 1. Both groups had similar numbers of intraglomerular and interstitial total leukocytes, monocytes/macrophages, and T cells (P = NS). However, glomerular LFA-1 alpha- and -beta-positive cells were significantly higher in group 1 (2.3 +/- 0.2 and 3.3 +/- 0.1 per glomerulus) than in group 2 (0.2 +/- 0.08 and 0.5 +/- 0.05 per glomerulus) (P < 0.005 and P < 0.01, respectively). Group 1 had much higher interstitial LFA-1 alpha- (109 +/- 20/mm(2)) and -beta-positive cells (157 +/- 40/mm(2)) in comparison with group 2 (29 +/- 12/mm(2) and 42 +/- 17/mm(2)). (P < 0.005 and P < 0.01, respectively). No association between glomerular and interstitial LFA-1-positive cells was seen. In addition, tubular HLA-DR expression was higher in group 1 (29 +/- 6/mm(2)) than in group 2 (9 +/- 2/mm(2)) (P < 0.005), but the interstitial HLA-DR-positive cells were similar in both groups. There was a significant association between interstitial LFA-1 alpha- and -beta-positive cells and tubular HLA-DR expression in group 1 (P < 0.01 and P < 0.005, respectively) but not in group 2. Interestingly, the extent of interstitial but not glomerular LFA-1-alpha and -beta expression was highly related to the degree of proteinuria in group 1 (P < 0.01 and P < 0.002, respectively). In conclusion, proteinuria in immunoglobulin A nephropathy is associated with increased LFA-1 expression by glomerular and interstitial infiltrating cells. However, interstitial but not glomerular LFA-1-positive cells are strongly related with the degree of urinary protein excretion. The exact link between LFA-1 expression and proteinuria needs further investigation.