Objective: To compare the effect of the combination rosiglitazone/metformin with the metformin as monotherapy, on the components of the metabolic syndrome (MS). Methods: A double blind, randomized, clinical study was carried out. Twenty five patients of both sexes, between 20 and 75 years of age, with the diagnosis of MS (ATP III criteria) were included. Blood pressure (BP), height, weight and abdominal circumference (AC) were registered. Fasting and 2 hours post load plasma glucose and insulin levels and fasting lipid profile were measured. The body mass index (BMI) and the HOMA index were calculated. The patients were distributed at random in two groups: Group A, 11 patients (10 women and 1 man), average age 43,45±14,98 who received metformin (500 mg) in combination with rosiglitazone (2 mg) in a tablet two times a day; Group B, 14 patients (13 women and 1 man), average age 44,21±10,28 who received metformin 850 mg in a tablet two times a day. Results: The studied variables were not different between both groups, before the treatment. With both treatments, the BMI and the AC diminished significantly though the percentage of change was bigger in the group A (BMI: 8,1 % versus 3,4 %; AC: 9,7 % versus 4 %; p <0,05). There was a significant decrease of the HOMA index and the fasting and post load insulin levels in the group A. In the group B was observed significant decrease of the values of BP and of the fasting and postload insulin levels and a not significant improvement of the HOMA index. Significant changes were not observed in the lipid profile. There was a positive correlation of the AC with the HOMA index (r=0,618; p=0,0007). Conclusions: Both therapies were beneficial in the treatment of the patients with MS. A major reduction of the AC and of the HOMA index in those who received the combination rosiglitazone/metformin and a major reduction of the values of blood pressure in those who received metformin as monotherapy were observed.