Increased plasminogen activator inhibitor-1 (PAI-1) activity has been reported in Type 2 (non-insulin-dependent) diabetes and is a recognized risk factor for coronary artery disease. Fourteen newly diagnosed Type 2 diabetic patients were studied before and 3 months after standard clinical dietary modification. To -assess the effect of improved metabolic control on PAI-1 activity, nine Type 2 diabetic patients established on diet therapy and with previous,stable glycaemic control served as controls. In the newly diagnosed patients diet therapy resulted in a significant decrease in HbA1c levels (8.3 +/- 0.5 vs 5.2 +/- 0.3 % (mean +/- SEM); p < 0.001), and this was accompanied by a fall in fibrinogen (4.3 +/- 0.3 vs 3.0 +/- 0.2 g.l-1; p < 0.01) concentration, and PAI-1 (18.7 +/- 2.3 vs 12.2 +/- 0.9 arbitrary units ml-1; p < 0.02) and factor VIII (147 +/- 17 vs 115 +/- 13 %; p < 0.01) activities. PAI-1 activity was correlated with triglyceride levels at the first assessment in the newly diagnosed patients (r = 0.66; p < 0.01), and this was the only independent association by multiple regression analysis when all patients (n = 23) were considered (r = 0.62; p < 0.002). However, there was no association between the changes in PAI-1 activity and the changes in HbA1c BMI, and serum triglyceride levels following treatment in the newly diagnosed patients. Serum triglyceride concentrations, HBA1c, PAI-1 activity, and the coagulation factors remained unchanged in the control group over the same treatment period. In conclusion, the introduction of conventional diet therapy in newly diagnosed Type 2 diabetic patients was accompanied by a significant improvement in circulating PAI-1 activity, but this could not be related to changes in the common indices of metabolic control.