The prevalence of preterm delivery is considerably elevated in women with type 1 diabetes. The aim of the study was to evaluate haemoglobin A(1c) (HbA(1c)) as a predictor of preterm delivery. Two hundred thirteen consecutive pregnant women with type 1 diabetes and normal urinary albumin excretion were included prospectively. HbA(1c) was analyzed at 10, 20 and 28 weeks of gestation. Seventy-one women (33%) delivered pre term and 142 at term. At 10 weeks of gestation, HbA(1c) was 7.3% (S.D. 1.0) vs. 6.9% (S.D. 0.9) (P<.01), at 20 weeks of gestation 6.6% (S.D. 0.7) vs. 6.1% (S.D. 0.7) (P<.001) and at 28 weeks of gestation 6.7% (S.D. 0.8) vs. 6.1% (S.D. 0.7) (P<.001). When comparing HbA(1c) at 10, 20 and 28 weeks of gestation, HbA(1c) at 28 weeks of gestation (P<.001) was the best predictor of preterm delivery. The adjusted odds ratio per 1% increment in HbA(1c) at 28 weeks of gestation was 2.8 (95% CI 1.7-4.4). HbA(1c) at 28 weeks of gestation was a clinical significant predictor of preterm delivery in type 1 diabetes.