BACKGROUND: A growing number of European studies, particularly from Nordic countries, suggest an increased frequency of autism in children of immigrant parents. In contrast, North American studies tend to conclude that neither maternal ethnicity nor immigrant status are related to the rate of autism-spectrum disorders. AIMS: To examine the hypotheses that maternal ethnicity and/or immigration are linked to the rate of childhood autism-spectrum disorders. METHOD: Retrospective case-note analysis of all 428 children diagnosed with autism-spectrum disorders presenting to the child development services in two centres during a 6-year period. RESULTS: Mothers born outside Europe had a significantly higher risk of having a child with an autism-spectrum disorder compared with those born in the UK, with the highest risk observed for the Caribbean group (relative risks (RRs) in the two centres: RR = 10.01, 95% CI 5.53-18.1 and RR = 8.89, 95% CI 5.08-15.5). Mothers of Black ethnicity had a significantly higher risk compared with White mothers (RR = 8.28, 95% CI 5.41-12.7 and RR = 3.84, 95% CI 2.93-5.02). Analysis of ethnicity and immigration factors together suggests the increased risk is predominately related to immigration. CONCLUSIONS: Maternal immigration is associated with substantial increased risk of autism-spectrum disorders with differential risk according to different region of birth and possibly ethnicity.