To determine the pathophysiology of gestational diabetes (GDM) in lean Japanese pregnant women in relation to insulin secretion or insulin resistance. The 75-g oral glucose tolerance test (OGTT) was performed in case of positive results of universal screening of a 50-g glucose challenge test at 24-28 weeks' gestation in Japanese pregnant women. These women were treated in our hospital between 2012 and 2016. Among these women, 30 with a body mass index of < 18.5 kg/m 2 were selected as lean subjects. Nine women were diagnosed with GDM (GDM group) and the remaining 21 had normal glucose tolerance (control group). For evaluating insulin secretion or resistance, the following parameters were compared between the two groups together with a family history of diabetes mellitus (DM) among first-degree relatives: (1) plasma glucose and immnunoreactive insulin (IRI) levels after glucose loading, (2) insulinogenic index (I.I), (3) homeostasis model assessment of β-cell function (HOMA-β), (4) homeostasis model assessment of insulin resistance (HOMA-IR), and (5) insulin sensitivity index (ISI) composite. The percentage of having a family history of DM was significantly higher in the GDM group (3/9, 33.3%) than in the control group (0/21, 0.0%, P < 0.001). Serum glucose levels at 30, 60, and 120 min after glucose loading were significantly higher in the GDM group than in the control group (all P < 0.05). IRI levels at 60 and 120 min were significantly higher in the GDM group than in the control group (both P < 0.05), and they showed persistent insulin secretion patterns. Values of the I.I. and ISI composite were significantly lower in the GDM group than in the control group (both P < 0.05), with no differences in HOMA-β, HOMA-IR and HbA1c levels between the groups. Lean Japanese pregnant women with GDM have impaired β-cell function, which is in part associated with hereditary traits.