The aim of this study is to explore beta cell dysfunction and its clinical significance in gestational diabetes mellitus (GDM). We assessed insulin sensitivity and insulin secretion in a total of 277 Japanese women between 24 and 27 weeks of pregnancy who underwent a 2 h, 75 g oral glucose tolerance test (OGTT) because of an abnormal result on a 1 h 50 g oral glucose challenge conducted as part of a standard screening for GDM. Insulin sensitivity was evaluated by an insulin sensitivity index derived from OGTT (IS_OGTT), whereas insulin secretion was calculated as a ratio of the total area under the insulin curve to the total area under the glucose curve (AUC_ins/glu). Beta cell function in relation to insulin sensitivity (i.e. disposition index) was derived from the product of insulin sensitivity and insulin secretion (i.e. AUC_ins/glu × IS_OGTT). In women diagnosed with GDM (n=57), the disposition index was significantly lower than that in those without GDM, irrespective of obesity. The disposition index in women with GDM was significantly correlated with levels of fasting and mean preprandial capillary glucose and HbA1c before initiating insulin therapy (r = -0.45, -0.38, -0.49, respectively). Furthermore, there was a significant correlation between the disposition index and total insulin dosage to achieve glycemic goal (r = -0.41). In conclusion, we demonstrated beta cell dysfunction in Japanese women with GDM irrespective of obesity. The level of beta cell dysfunction in GDM was associated with the severity of glucose intolerance and total insulin dosage required. These findings underpin clinical significance of beta cell dysfunction in GDM.