A European Community concerted action charged with assessing AIDS prevention for migrants and/or ethnic minorities raised the problem of the quality and possibility for misuse of existing HIV and AIDS data. Basing statistics on the number of foreigners in a country is problematical as numbers are affected by variations in definitions and policies concerning immigration. Categorizing by ethnic origin raises serious definitional problems. For the numerator, epidemiology must be based on AIDS case data since systematic HIV testing of migrants is excluded on both practical and human rights grounds, but there are reasons for both over- and under-reporting of AIDS in migrant groups. Underlying issues of stigmatization and of racism are discussed. While there Is need for improvement in the epidemiological data collected, both planning and evaluation of HIV/AIDS prevention programmes should more reasonably be based on proxy indicators, essentially those of knowledge, attitudes and behaviours, as well as on good ethnographies.