We evaluated the relationship between plasma fibrinogen concentration and the serum levels of interleukin-6 (IL-6), its soluble receptor, and their complex in patients with type 2 diabetes mellitus. The study comprised 57 patients with type 2 diabetes and 15 normal healthy controls. Serum levels of IL-6, soluble IL-6 receptor (IL-6R), and circulating IL-6/IL-6R complex were determined by enzyme-linked immunosorbent assays. Correlations between the different study parameters and serum IL-6, IL-6R, or IL-6/IL-6R complex levels were determined by multiple linear regression analysis. Any association between the different study parameters and the serum levels of IL-6, IL-6R, or IL-6/IL-6R complex were determined by stepwise linear regression analysis. The serum IL-6 level in diabetic subjects was significantly higher than in normal healthy controls (3.48 +/- 3.29 pg/ml vs 0.784 +/- 0.90 pg/ml, mean +/- SD, respectively, P = 0.0001). The specific optical density of the serum IL-6/IL-6R complex in diabetic patients was also significantly higher than in normal healthy controls, although there was no significant difference in the serum IL-6R level between diabetic patients and controls. The serum IL-6 concentration was correlated significantly with the HbA(1C) level (beta = 0.58, P = 0. 04) by multiple regression analysis. Stepwise regression analysis revealed that the levels of serum IL-6 (F = 8.251), HbA(1C) (F = 1. 08), and serum urea nitrogen (F = 5.603) were associated with the plasma fibrino gen concentration. These results suggest that hyperglycaemia and increased levels of serum IL-6 can increase the plasma fibrinogen concentration, one of the known risk factors for atherosclerosis in patients with type 2 diabetes mellitus.