The objective of this study was to investigate the utility and characteristics of various screening procedures for gestational diabetes mellitus (GDM) in Japan during the first trimester and between 24 and 28 weeks of pregnancy. The subjects were 749 pregnant women who came to our hospitals. A 50-g oral glucose challenge test (GCT), casual plasma glucose measurements, fasting blood glucose measurements, and glycosylated hemoglobin measurements were performed in the first trimester. Subjects with no abnormalities were tested again at 24-28 weeks of gestation. Of the 749 subjects, 22 (2.9%) tested positive for GDM. Of those 22 patients, 14 were diagnosed with GDM in the first trimester (63.6%) and eight in the second trimester (36.4%). This finding suggests the importance of screening for glucose intolerance in the first trimester. Furthermore, it appears that the GCT has the most utility for GDM screening; the other screening methods tested were not as useful because of their low sensitivity, particularly in the second trimester.