The frequency of abnormal glucose tolerance in the first 12 months after gestational diabetes was found to be 33.3%, which is much higher than previously accepted. Women with gestational diabetes (Group 1 = 54 requiring insulin, Group 2 = 32 treated with diet alone) attending a metropolitan teaching hospital over a 3 1/2 year period were followed-up after delivery to determine their subsequent glucose tolerance. Of 86 seen 3 months after delivery, 2 had developed insulin-dependent diabetes mellitus (IDDM) and 2 noninsulin dependent diabetes mellitus (NIDDM), diagnosed by glucose tolerance testing. Another 38 returned for follow-up glucose tolerance testing at 12 months; of these 3 had impaired glucose tolerance (IGT), 7 had NIDDM, and one who had had NIDDM at 3 months now showed IGT after 9 months dietary treatment. Thus, 12 months after delivery, the cumulative prevalence of abnormal glucose tolerance was 14/42 (33.3%), 10 of the 42 being frankly diabetic (26%). Of the remaining 44 patients, 21 have not yet reached 12 months or were pregnant again, and 23 did not attend for glucose tolerance testing. Although the trend was for gestational diabetes mellitus (GDM) to recur earlier and more severely in subsequent pregnancies, in 3 instances the diabetes did not recur. Major congenital malformations occurred in 4 of the 86 babies (4.7%); minor malformations were found in a further 13 (15%) with no difference in frequency between Group 1 and Group 2.