Objective. We evaluated screening with a diagnostic oral glucose tolerance test earlier than 20 weeks gestation in women with moderate to severe obesity. Design. Prospective observational study. Setting. Large university teaching hospital. Population. We enrolled 100 women booking for antenatal care in the first trimester at their convenience. Methods. Height and weight were measured and body mass index calculated. Only women with a body mass index>34.9 kg/m2 were included. Women were booked for a 100 g oral glucose tolerance test before 20 weeks and, if normal, another test at 28 weeks gestation. Main outcome measures. Impaired glucose tolerance and gestational diabetes mellitus. Results. Of the 100 women given an appointment for an oral glucose tolerance test before 20 weeks gestation, 92 attended. Of these, 10 (10.8%) women had an abnormal result, with impaired glucose tolerance in five (5.4%) cases and gestational diabetes mellitus in five (5.4%) cases. Of those with a normal result at 20 weeks, 81 attended for a repeat test at 28 weeks gestation. A further four (4.9%) had impaired glucose tolerance and four (4.9%) had gestational diabetes mellitus. A total of 18 (20.5%) of the 88 women who complied with screening had an abnormal test. Conclusions. Women who have moderate/severe obesity have a one in five chance of having an abnormal diagnostic oral glucose tolerance test when screened for gestational diabetes mellitus. To optimize maternal glycemic control in pregnancy, we suggest that women with a body mass index>34.9 kg/m2 may need to be screened early in pregnancy and, if the test is normal, again at 28 weeks gestation.