To study fibrinolysis in relation to microvascular diabetic complications, 20 control subjects were compared with 50 Type 1 (insulin-dependent) diabetic patients of similar age, 20 with no complications, 17 with laser-treated retinopathy, and 13 with neuropathy and retinopathy. None were smokers, hypertensive or had macrovascular disease. Pre- and post-venous occlusion blood samples for tests of fibrinolysis were taken. Median (interquartile range) basal tissue plasminogen activator (t-PA) activity was lower in control subjects (100 (less than 100-100) IU l-1) than diabetic patients (uncomplicated 145 (100-280) IU l-1, p = 0.015; retinopathy 180 (100-228) IU l-1, p = 0.037; neuropathy 210 (125-310) IU l-1, p = 0.004, respectively). Basal t-PA inhibition (PAl-1 activity) was higher in control subjects (5.9 (4.5-9.5) kIU l-1) than diabetic patients (uncomplicated 4.0 (3.3-5.0) kIU l-1, p = 0.001; retinopathy 4.5 (3.1-6.3) kIU l-1, p = 0.058; neuropathy 4.0 (3.0-5.4) kIU l-1, p = 0.015, respectively). Post-venous occlusion t-PA antigen was higher in control subjects (10.2 (7.3-15.1) micrograms l-1) than neuropathic patients (5.5 (4.9-7.3) micrograms l-1, p = 0.004). Other tests showed a consistent, but non-significant, trend towards increased basal fibrinolysis in the Type 1 diabetic patients. The results indicate that Type 1 diabetic patients have enhanced basal fibrinolysis. The diminished response to venous occlusion in neuropathic patients is consistent with an endothelial cell defect.(ABSTRACT TRUNCATED AT 250 WORDS)