Platelets are involved in atherogenesis, partly through the release of smooth muscle cell mitogens and chemoattractants such as platelet-derived growth factor (PDGF). The St Thomas' Atherosclerosis Regression Study (STARS) demonstrated that a cholesterol-lowering diet induced angiographic regression of coronary artery disease in 74 men. Serum PDGF concentrations were not different at the end of the study between patients randomised to receive diet, with or without cholestyramine: usual care (UC) median 53.6 pM (interquartile range 15.3), diet (D) 60.0 pM (29.7), diet and cholestyramine (DC) 51.5 pM (13.0), 2p = 0.23. Similarly, mean platelet volume (MPV), a marker of platelet function, did not differ between the three groups: UC 8.0 fl (1.1), D 8.6 fl (0.8), DC 9.0 fl (1.4), 2p = 0.16. PDGF concentrations and MPV did not correlate with lipid concentrations or angiographic indices of regression. These findings suggest that platelet function, as measured by PDGF and MPV, does not change during, or contribute to, dietary-induced regression of coronary artery disease.