The incidence of Type 1 (insulin-dependent) diabetes mellitus among Moroccan children aged (0-19 years) in The Netherlands was determined. Point of reference was the data derived from the second nationwide incidence study on Type 1 diabetes among children under 20 years of age. In that study the incidence among Dutch children was 13.2 100000-1 year-1. To scrutinize the data and to obtain more information a questionnaire was sent in 1993 to all specialists who had reported that they had diagnosed a patient with Type 1 diabetes during the years 1988-1990 whose parents originated from Morocco, Turkey or other foreign countries. The questionnaire requested information on origin and migration of child and parents. The response to the questionnaire was 86% for the Moroccan children, 75% for the Turkish children and 100% for the children from other countries. In only one case a wrong country had been recorded. None of the patients had been in The Netherlands for less than 6 months before the diagnosis. The incidence for Moroccan children was 20.0 (95% CI 14.6-26.9) and for Turkish children 4.5 (95% CI 2.2-8.0) 100000-1 year-1. It is concluded that the incidence of Type 1 diabetes in Moroccan children (0-19 years) is 1.5 times higher than in Dutch children and 4.5 times higher than in Turkish children.