Platelet volume, indicating platelet activity has been proposed as a further independent risk factor for cardiovascular disease (CVD). It is well established that weight reduction reduces some of the risk factors for CVD in obese patients. We therefore retrospectively investigated the effects of weight reduction on platelet mass, platelet volume and serum lipid profile in patients undergoing an eight-week weight reduction therapy. During weight reduction, the body mass index (P < 0.005), serum total cholesterol (P < 0.05), LDL cholesterol (P < 0.01) and serum triglycerides (P < 0.01) were significantly reduced, while HDL cholesterol remained unchanged. On the other hand, platelet mass (P < 0.005) and platelet volume (P < 0.0005) increased significantly within the same period. We therefore conclude that fasting might alter the control of platelet size with a possible impact on platelet activity. This might result in an increased risk of thrombo-embolic ischaemic events in atherosclerotic patients.