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Summary. We examined the relationship between diabetes in pregnancy and the development of congenital malformations, in a population-based retrospective study using birth certificate data for all livebom children delivered from 1984 until 1991 in Washington State. Births to mothers with established and gestational diabetes numbered 1511 and 8869 respectively. For comparison we selected 8934 births to mothers without diabetes. The prevalence of congenital malformations in neonates was 7.2%, 2.8% and 2.1% among mothers with established diabetes, gestational diabetes, and without diabetes, respectively. Newborns of mothers with established diabetes were more likely to have a congenital malformation than newborns of non-diabetic mothers (prevalence odds ratio = 4.0; 95% confidence interval 3.1–5.1). In contrast, there was only a slightly higher prevalence of congenital malformations among newborns of mothers with gestational diabetes (prevalence odds ratio = 1.3; 95% CI 1.0–1.6). The association with maternal established diabetes was greater for neonates with multiple malformations (7.8; 95% CI 3.3–18.1) than for single malformations (2.9–95% CI 2.1–3.9). Four to seven fold associations were observed with skeletal, cleft lip/palate, neural tube and heart abnormalities. The association of established diabetes with congenital malformations was nearly twice as strong among female neonates (prevalence odds ratio = 5.4; 95% CI 3.7–8.0) than among male neonates (prevalence odds ratio = 3.1; 95% CI 2.2–4.5). No such variation was observed for associations with gestational diabetes. This study enlarges on previous work relating congenital anomalies to established diabetes and supports the possibility of a weak association with gestational diabetes.