Background: As in other Western countries, the number of immigrants in the Netherlands is growing rapidly. In 1980 non-western immigrants constituted about 3% of the population, in 1990 it was 6% and currently it is more than 10%. Nearly half of the migrant population lives in the four major cities. In the municipality of Rotterdam 34% of the inhabitants are migrants. Health policy is based on the ideal that all inhabitants should have equal access to health care and this requires an efficient planning of health care resources, like staff and required time per patient. The aim of this study is to examine ethnic differences in the use of internal medicine outpatient care, specifically to examine ethnic differences in the reason for referral and diagnosis. Methods: We conducted a study with an open cohort design. We registered the ethnicity, sex, age, referral reasons, diagnosis and living area of all new patients that visited the internal medicine outpatient clinic of the Erasmus Medical Centre in Rotterdam (Erasmus MC) for one year (March 2002-2003). Additionally, we coded referrals according to the International Classification of Primary Care (ICPC) and categorised diagnosis according to the Diagnosis Treatment Combination (DTC). We analysed data by using Poisson regression and logistic regression. Results: All ethnic minority groups (Surinam, Turkish, Moroccan, Antillean/Aruban and Cape Verdean immigrants) living in Rotterdam municipality, make significantly more use of the outpatient clinic than native Dutch people (relative risk versus native Dutch people was 1.83, 1.97, 1.79, 1.65 and 1.88, respectively). Immigrant patients are more likely to be referred for analysis and treatment of 'gastro-intestinal signs & symptoms' and were less often referred for 'indefinite, general signs'. Ethnic minorities were more frequently diagnosed with 'Liver diseases', and less often with 'Analysis without diagnosis'. The increased use of the outpatient facilities seems to be restricted to first-generation immigrants, and is mainly based on a higher risk of being referred with 'gastro-intestinal signs & symptoms'. Conclusion: These findings demonstrate substantial ethnic differences in the use of the outpatient care facilities. Ethnic differences may decrease in the future when the proportion of first-generation immigrants decreases. The increased use of outpatient health care seems to be related to ethnic background and the generation of the immigrants rather than to socio-economic status. Further study is needed to establish this.