Postmenopausal women with the metabolic syndrome are at high risk of oxidative stress. Several studies have suggested possible antioxidant properties of soya, but little evidence is available regarding the effect of soya on oxidative stress in postmenopausal women with the metabolic syndrome. The objective of the present study was to determine the effects of soya consumption on plasma total antioxidant capacity (TAC) and malondialdehyde (MDA) level in postmenopausal women with the metabolic syndrome. A randomised cross-over trial was undertaken on forty-two postmenopausal women with the metabolic syndrome. Participants were randomly assigned to consume a control (Dietary Approaches to Stop Hypertension; DASH) diet, a soya protein diet, or a soya nut diet, each for 8 weeks. Red meat in the DASH diet (one serving per d) was replaced by soya protein in the soya protein period and by soya nuts in the soya nut period. Significant differences between the end values of the control diet, soya protein diet and soya nut diet were seen for MDA (0·70, 0·64 and 0·63 μmol/l; global P < 0·01). The results also showed a significant difference between the end values for TAC (1950, 2030 and 2110 μmol/l, respectively; P < 0·01). The difference from control for TAC was +4·5 % (P < 0·01) in the soya nut period and +5·8 % (P < 0·01) in the soya protein regimen. Both soya nuts and soya protein decreased MDA significantly compared with the control diet (difference from control was − 7·9 % (P < 0·01) in the soya nut period and − 9·4 % (P < 0·01) in the soya protein diet). We conclude that soya consumption reduces plasma MDA and increases plasma TAC levels in postmenopausal women with the metabolic syndrome.