Plasma fibrinogen is accepted as an important risk factor for atherosclerosis, stroke, coronary heart disease (CHD) and the cardiovascular complications of some diseases such as diabetes mellitus and hyperlipidaemia. However, it is suspected that not only fibrinogen concentration, but also the quality of fibrin clots may contribute to CHD risk. Very little is known about the contribution of diet to plasma fibrinogen concentration in human subjects and no data are available on the effect of diet on fibrin clot structure properties. The main aim of these studies was to investigate the possible effect of diet on fibrin clot network properties in vivo. Firstly, fibrin clot structure properties were measured in two paired/ matched non-insulin dependent diabetes mellitus subject groups (=8 per group), following an energy restricted highfibre, low-fat, high-carbohydrate diet, supplemented with either 150 g dry beans (cooked weight), containing plant protein and soluble fibre, or 50 g soy beans (raw weight), containing plant protein without fibre, per day. Supplementation was sustained for 10 weeks. A significant increase in the mass to length ratio of fibrin fibres (mT) (from 13.15±1.08to 14.58±2.14daltons/cmx 1012) were measured in the subject group receiving the plant protein supplement. Improvement of diabetes management was also shown by significant reduction of glycosylated haemoglobin (from 10.83±2.36 to 8.23±1.42%) in the same group. From the results of this study it is suspected that the observed changes in fibrin clot structure properties were mainly mediated by improvement in diabetes management. Secondly, the effect of the soluble dietary fibre pectin, known for its hypocholesterolaemic properties, on fibrin clot structure properties were measured in hyperlipidaemic subjects. Effects of the pectin supplement (15 g per day for 4 weeks) were measured against a corn flour placebo. Pectin supplementation caused significant decreases in both the clot fibrin content (from 2.22±0.47 to 1.90±0.17 g/L) and clot lysis time (from 285.6±16.1 to 232.9± 17.9 min). Clot compaction was significantly increased (from 21.51 ±3.65 to 30.16±4.41°/o), as well as mT (from 19.94±6.27 to 24.80±4.22 daltons/cm x 1012) and clot permeability (from 279.6±101.2 to 336.3±119.1 x 1011 cm2). A significant decrease in total cholesterol (TC) (from 6.50±0.27 to 5.67±0.48 mmol/L) was also observed. The results of this study suggest that changes in fibrin clot structure properties may be concomitant with changes in other CHD risk variables, such as TC. Conclusion: From results of both intervention studies, it is evident that fibrin clot structure can be modified by dietary intervention. It is known that fibrin clot structure properties are partially determined during the clotting process by the characteristics of the direct surrounding. Diet may alter the direct surrounding of the fibrinogen molecule, and thus alter fibrin clot structure by indirect mechanisms. © Pearson Professional Ltd 1996.