The biological activity of thrombin and coagulation factor Xa was assessed in 62 insulin-dependent diabetic patients. A group of non-diabetic subjects of comparable age and urinary albumin excretion rate (< 30 mg/24 h) served as control subjects (group 1, n = 14). The patients were divided into three groups according to urinary albumin excretion rate. In group 2, albumin excretion rate was less than 30 mg/24 h (n = 17), in group 3 albumin excretion rate was in the range 30-300 mg/24 h (n = 20) and in group 4 albumin excretion rate was greater than 300 mg/24 h (n = 25). Compared to non-diabetic control subjects an increase in the biological activity of factor Xa was observed in all groups of diabetic patients (prothrombin fragment 1 + 2 levels were 1.14 +/- 0.38 nmol/l in group 2, p < 0.005; 1.06 +/- 0.45 nmol/l in group 3, p < 0.05 and 1.03 +/- 0.31 nmol/l in group 4, p < 0.05 vs 0.75 +/- 0.34 nmol/l in group 1). No difference in the level of antithrombin III was seen between the groups. We reconfirmed the presence of intimal dysfunction in diabetic nephropathy demonstrated by elevated transcapillary escape rate of albumin in group 4 compared with group 2 (8.9 +/- 2.0% vs 7.0 +/- 1.9%, p < 0.05). An overall positive correlation between transcapillary escape rate and prothrombin fragment 1 + 2 was found (r = 0.36, p < 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)