In 138 oligo-amenorrheic white women with polycystic ovary syndrome (PCOS) (31+/-9-years-old), our first specific aim was to assess the incidence of the metabolic syndrome and to compare metabolic syndrome abnormalities in women with PCOS to those in the National Health and Nutrition Examination Survey (NHANES) III cohort of 1,887 white women. Our second aim was to determine whether metformin (2.55 g/d) and a diet of 1,500 calories, 26% protein, 44% carbohydrate (42% of carbohydrate complex), 30% fat (polyunsaturate/saturate ratio [P/S]=2/1), would ameliorate metabolic syndrome abnormalities in women with both PCOS and metabolic syndrome. The metabolic syndrome was present in 64 (46%) of the women with PCOS. In these 64 women, there were abnormalities in waist circumference (98%), high-density lipoprotein cholesterol (HDL-C) (95%), blood pressure (70%), triglycerides (56%), and glucose (11%). In these 64 women, mean +/- SD waist circumference was 116+/-15 cm, triglyceride 192+/-152 mg/dL, HDL-C 39+/-7 mg/dL, systolic blood pressure 131+/-13 mm Hg, diastolic blood pressure 83+/-7 mm Hg, and serum glucose 94+/-22 mg/dL. Serum insulin was high (>17 microU/mL) in 42 of the 64 women (66%). After age adjustment, 46.4%+/-4.2% of women with PCOS had the metabolic syndrome (> or =3 abnormalities) versus 22.8%+/-1.1% of NHANES III women, P<.0001 versus 6% of 20 to 29-year-old and 15% of 30 to 39-year-old NHANES III women. Of the 64 women with both PCOS and the metabolic syndrome, 50 had follow-up studies after an average of 6 months on metformin and diet. At 6 months follow-up, mean percent reductions were as follows: body weight 4.7% (111 to 106 kg, P<.0001), triglycerides 14% (197 to 136 mg/dL, P=.0001), systolic blood pressure 5.2% (131 to 124 mm Hg, P=.0002), diastolic blood pressure 6% (83 to 77 mm Hg, P=.0007), and insulin 31% (25 to 17 microU/mL, P<.0001); mean percent HDL-C increased 6% (39 to 41 mg/dL, P=.013). Of these 50 women, 29 had pretreatment baseline abnormal triglycerides (> or =150 mg/dL), 47 had low HDL-C (<50 mg/dL), 26 had high systolic blood pressure (> or =130 mm Hg), 16 had high diastolic blood pressure (> or =85 mm Hg), and 5 had glucose > or = 110 mg/dL. On metformin plus diet at 6 months, triglycerides moved within guidelines in 10 of 29 (34%) women, HDL-C in 6 of 47 (13%), systolic blood pressure in 16 of 26 (62%), diastolic blood pressure in 10 of 16 (63%), and glucose in 3 of 5 (60%). Metformin and diet ameliorate many of the features of the metabolic syndrome, present in 46% of women with PCOS in the current study, and should reduce risk for atherothrombosis and type 2 diabetes mellitus (DM) in PCOS.