The 1945 census was the last to record ethnicity in Sweden, and in 1973, registering ethnicity became virtually illegal. Swedish population statistics and official health and social registers therefore do not contain information on the ethnic background of the people registered. Exploring socio-medical and epidemiological research in Sweden from the 1940s to the 1980s, the chapter discusses the tension between ethical and legal constraints on the one hand, and the scientific, administrative, and political need for accurate information on minority and ethnic groups on the other. Three alternative research strategies to substitute for the lack of data on ethnicity are identified: using alternative but related categories such as nationality, country of birth, or immigrant status; “reading ethnicity” from pre-existing data collections; and conducting time-consuming and costly special studies designed to enable researchers to bypass the proscription of ethnic registration. These strategies have produced new epidemiological and socio-medical understandings concerning ethnicity and health/disease. However, as certain groups and categories have been statistically more available or visible in society than others, an uneven body of knowledge has evolved. In effect, some minorities have been spotlighted while others have remained hidden in plain sight.