Plasma fibrinogen was measured in 285 diabetics (age range 15-85 years) and 209 controls (age range 23-74 years). Plasma fibrinogen concentration showed a positive skew distribution and an approximate normal distribution was obtained by log transformation. The mean log plasma fibrinogen in the diabetics was significantly increased (p less than 0.0001). Patients treated with a sulphonylurea (n = 81) had the highest mean log plasma fibrinogen concentration and this was significantly higher than in patients treated with insulin (n = 76; p less than 0.01), biguanides (n = 28; p less than 0.01) or sulphonyluera plus biguanides (n = 38; p less than 0.05). The biganide treated group had the lowest mean log plasma fibrinogen concentrations. No correlation was found between plasma fibrinogen and blood glucose, duration of diabetes or the presence of complications. Subjects with proliferative retinopathy (n = 38) had a similar mean plasma fibrinogen to those with background retinopathy (n = 55). Twenty-two maturity onset diabetics treated with a sulphonylurea and followed prospectively showed a significant increase in plasma fibrinogen after five months (p less than 0.0001), while a control diet treated group showed no alteration in plasma fibrinogen. It is concluded that plasma fibrinogen is significantly increased in diabetics and apart from age, the main factor related to the increase is treatment with sulphonylureas. It remains speculative whether this association has any correlation with long term cardiovascular morbidity.