Platelet‐derived growth factor (PDGF) and endothelin are both smooth muscle cell mitogens, and may have an important role in mediation of the proliferative phase of atherogenesis. Since hypercholesterolaemia is an important risk factor for atherosclerosis, we have measured serum endothelin and PDGF concentrations in matched pairs of hypercholesterolaemic patients and control subjects. Both endothelin and PDGF concentrations were higher in the hypercholesterolaemic patients (23.8±4.3 pm vs. 21.7±2.4pm, P = 0.019, median difference 3.0, 95% confidence limits 0.6–7.3, and 115.4±60.3 pm vs. 64.7±22.7 pm P = 0.033, median difference 27.2, 95% confidence limits 1.3–82.5, respectively), while serum endothelin and PDGF concentrations were positively correlated with each other (r = 0.764, P < 0.0001, 95% confidence limits 0.505–0.897). PDGF concentrations were correlated with the peripheral blood platelet count (r = 0.560, P = 0.007, 95% confidence limits 0.180–0.794). These findings suggest that PDGF, and possibly endothelin, may be involved in hypercholesterolaemia‐ related atherogenesis.