Obesity is a chronic metabolic disorder associated with cardiovascular disease and atherosclerosis. Platelet activation and aggregation are central processes in the pathophysiology of cardiovascular disease. Mean platelet volume (MPV), a determinant of platelet activation, is a newly emerging risk marker for atherothrombosis. Our objective was to evaluate the effect of weight loss on the MPV in obese patients. We selected 30 obese women patients and 30 non-obese healthy women subjects. All obese patients took the same content and caloric diet treatment for 3 months. Body mass index (BMI), metabolic parameters and MPV were measured at baseline and after 3 months diet treatment. Before diet treatment, obese group had significantly higher MPV levels than in the non-obese control group (8.18 +/- 1.09 fl vs. 8.01 +/- 0.95 fl, p = 0.004). MPV showed positive correlations with BMI level in the obese group (r = 0.43, p = 0.017). BMI significantly decreased after diet treatment (36.2 +/- 3.2 kg/m(2) vs. 34.7 +/- 3.6 kg/m(2), p < 0.001), in the obese group. MPV significantly decreased after diet treatment in the obese group (8.18 +/- 1.09 fl vs. 8.08 +/- 1.02 fl, p = 0.013). There was a positive correlation between weight loss and reduction in MPV (r = 0.41, p = 0.024). In addition to its well-known positive effects on cardiovascular disease risk, weight loss may also possess significant anti-platelet activation properties that can contribute its antiatherogenic effects in obese patients.