BACKGROUND: Resistin, leptin and adiponectin are proteins secreted from adipose tissue, and have been suggested to play roles in insulin sensitivity. The effects of the circulating levels of two different types of insulin sensitizer, rosiglitazone and metformin, in type 2 diabetic patients were examined to elucidate the relationship between adipokines and insulin resistance. METHODS: Thirty type 2 diabetic patients, who showed poor glycemic control when administered 4 mg glimepiride a day, without severe diabetic complications or medical illness, were randomized to receive an additional 4mg rosiglitazone or 1000 mg metformin a day. The plasma resistin, leptin and adiponectin concentrations were measured at the baseline and after 6 months of treatment. The anthropometric parameters, fasting plasma glucose, HbA1C, total cholesterol, triglyceride, HDL-cholesterol and free fatty acids were also measured. Certain single nucleotide polymorphisms of adipokine genes were also identified. RESULTS: There were no significant differences in the reductions of the plasma glucose and HbA1C levels, after 6 months of treatment, between the two groups. The plasma resistin concentrations decreased, the adiponectin significantly increased and the leptin showed a tendency to increase in the rosiglitazone group. In the metformin group, only the resistin concentration significantly increased. However, the changes in the adipokines did not correlate with the HOMA-IR in either group. The reduction in the HbA1C due to rosiglitazone was greater if the initial leptin level was high, if there was a G allele on the -420th locus of the resistin gene, or the 45th locus of the APM1 (adiponectin gene) was the T-homozygote or there was a T allele on the 276th locus of the APM1. Those due to metfromin were greater with high initial adiponectin levels. CONCLUSION: In type 2 diabetic patients, showing poor glycemic control with sulfonylurea therapy, rosiglitazone or metformin treatment changed some of the adipokine concentrations, but these changes were not clearly related with insulin resistance. Polymorphisms of certain adipokine genes seem to have a relation to the susceptibility of rosiglitazone.