Abnormalities of platelet aggregation and coagulation have been reported in insulin dependent diabetes mellitus (IDDM), although there is controversy concerning their relationship to microangiopathy. We have studied platelet function and haemostasis in 55 patients with IDDM, 23 without, 14 with mild (background retinopathy) and 18 with severe (proliferative retinopathy, or background retinopathy plus proteinuria) complications. Studies were done on 2 occasions 8 weeks apart and the results compared with 28 control subjects. There was evidence of increased in vivo platelet aggregation in the diabetic group v controls shown by raised values of beta-thromboglobulin (61 +/- 42, mean +/- SD, v 18 +/- 14 micrograms/ml, p less than 0.001), platelet factor 4 (62 +/- 76 v 14 +/- 11 micrograms/ml, p less than 0.01), and platelet micro-aggregates (20 +/- 16 v 12 +/- 11%, p less than 0.01). There was no significant difference in fibrinogen and fibrinopeptide A levels, nor in 'in vitro' tests of platelet aggregation between the groups. Dilute whole blood clot lysis time was increased in the diabetic group v controls (6.4 +/- 2.6 v 4.8 +/- 0.5 hours, respectively, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)