Tumor necrosis factor-alpha (TNF-alpha) system is potentially involved in the development of insulin resistance during pregnancy. Plasma concentrations of TNF-alpha and its soluble receptors sTNFR-1 and sTNFR-2 were measured in 80 patients with gestational diabetes (GDM) (mean age 29.0 +/- 4.9 years) and 30 pregnant women with normal glucose tolerance (NGT) (mean age 28.2 +/- 6.0 years). We found that patients with GDM had significantly higher levels of TNF-alpha in comparison to NGT women (1.71+/- 0.92 vs. 1.27 +/- 0.42 pg/ml, p = 0.0175). The differences remained statistically significant after adjusting for BMI (p = 0.027). Plasma levels of sTNFR-1 and sTNFR-2 were only slightly higher in patients with GDM (2.83 +/- 0.79 ng/ml vs. 2.55 +/- 0.99 ng/ ml, p = 0.057 and 7.46 +/- 2.21 ng/ml vs. 6.83 +/- 1.46 ng/ml, p=0.206, respectively). In the group with GDM TNF-alpha concentrations correlated with sTNFR-1 (r = 0.444, p = 0.00008), sTNFR-2 (r = 0.364, p = 0.0016) and with C-peptide concentrations (r = 0.318, p = 0.016), whereas in women with NGT - only with triglyceride levels (r = 0.50, p = 0.024). Multivariate linear regression analysis revealed that early pregnancy BMI was the most predictive indicator of TNF-alpha concentrations in GDM women (p=0.008). In NTG group triglyceride concentrations, as well as BMI in early pregnancy and at the time of sampling were significant predictors, explaining together 62% of the variance in TNF-alpha concentration. In conclusion, increased TNF-alpha concentrations in women with GDM class G1 indicates its contribution to the development of insulin resistance during pregnancy, but the lack of the differences in sTNFR concentrations between the groups studied suggests only moderate TNF-alpha system activation in relatively slim patients treated with diet.