Studies have shown that polymorphisms located at positions -106 and approximately -2100 base pairs (5'ALR2) in the regulatory region of the aldose reductase gene are associated with susceptibility to microvascular complications in patients with diabetes. The aim was to investigate the functional roles of these susceptibility alleles using an in vitro gene reporter assay. Susceptibility, neutral and protective 5'ALR2/-106 alleles were transfected into HepG2 cells and exposed to excess D-glucose (D-glucose at final concentrations 14 or 28 mmol/l). Transcriptional activities were determined using a dual luciferase reporter gene assay. The "susceptibility alleles" Z-2 with C-106 had the highest transcriptional activity when compared with the "protective" combination of Z+2 with C-106 alleles (58.7+/-9.9 vs. 10.1+/-0.7; P<0.0001). Those constructs with either the Z or Z-2 in combination with the C-106 allele had significantly higher transcriptional activities when compared to those with the T-106 allele (Z/C-106, 37.4+/-5.4 vs. Z/T-106 7.7+/-1.6, P<0.003; Z-2/C-106, 58.7+/-9.9 vs. Z-2/T-106 10.9+/-0.6, P<0.0001). These results demonstrate that the Z-2/C-106 haplotype is associated with elevated transcriptional activity of the aldose reductase gene. This in turn may explain the role of these polymorphisms in the susceptibility to diabetic microvascular complications.