Congenital malformations are among the major causes of perinatal mortality and morbidity at present. Research into the ethnic diversity of congenital malformations can form a basis both for aetiological studies and for health care advice and planning. This study compared the overall prevalence of congenital malformations, the prevalence in different organ systems and of several specific malformations between different maternal ethnic groups in the Netherlands using a 5-year national birth cohort (1996-2000) containing 881 800 births. Maternal ethnic groups considered were Dutch; Mediterranean (Moroccan/Turkish); other European; Black; Hindu and Asian. Mediterranean women had a 20% higher risk of having a child with a congenital malformation than Dutch women (age-adjusted OR = 1.21 [95% CI 1.16, 1.27]). They showed an increased risk of malformations in several organ systems such as the central nervous system and sensory organs, the urogenital system and skin and abdominal wall. Further, they had an increased risk of the group of chromosomal malformations/multiple malformations/syndromes. For the specific group of multiple malformations the maternal age adjusted OR was 1.80 [95% CI 1.47, 2.20]. The Black group showed a significantly increased risk of skeletal and muscular malformations (age adjusted OR = 1.76 [95% CI 1.53, 2.02]) with a sixfold increased risk of polydactyly compared with the Dutch group. For Mediterranean women, the largest and fastest growing group of immigrants in the Netherlands, this study demonstrated an increased risk of congenital malformations.