A low-grade systemic inflammation is concomitant in diabetes. There is a pathophysiological relation between gestational diabetes mellitus and type 2 diabetes mellitus, which was further supported by significantly elevated risk of type 2 diabetes in women with a history of previous gestational diabetes mellitus. We investigated the relation between low-grade systemic inflammation expressed as C-reactive protein and gestational diabetes in non-obese pregnant women. This study included 20 non-obese pregnant women with gestational diabetes mellitus and 30 non-obese pregnant women without gestational diabetes mellitus as a control group. The body mass index of all the subjects were < 25 kg/m2. During 26-28 gestational weeks 100-g oral glucose tolerance test was performed and simultaneously fasting C-reactive protein levels were measured. Serum median C-reactive protein level was higher in patients with gestational diabetes mellitus (p = 0.0001). C-reactive protein was strongly associated with glycemic parameters and weight gain during pregnancy. A model consisting of glucose intolerance, age, parity, and weight gain during pregnancy accounted for 61% of the variance in log C-reactive protein. We demonstrated that serum C-reactive protein level was related with gestational diabetes mellitus and weight gain during pregnancy in late second and early third trimesters.