"Ethnic group" is a problematic variable in health-related research. While self-identification is now widely accepted as the appropriate mode of assignment, the impracticalities of a free response in the collection of ethnic group data mean that categorisation into a limited set of choices must take place. The substantial and increasing number of persons in minority ethnic groups who identify through non-standard responses emphasises the need to develop classifications that accommodate salient vernacular terminology. The use of informants in cognitive settings and the monitoring of open-ended responses appear to offer the best way of determining which group labels to employ. The recommended approach addresses the research priorities for accurate, consistent, and high quality data and is also responsive to the dynamic nature of ethnic group and the growing ethnic diversity of the population.