Minor histocompatibility antigens (MiHAs) are extremely numerous - possibly up to 720 in mice - and have been regarded as an 'immunogenetic nightmare' in transplantation. Here, Claude Perreault, Denis Roy and Claudette Fortin summarize evidence showing that most anti-MiHA T-cell responses are oligoclonal, being restricted to several dominant MiHAs, rather than polyclonal. This concept has important implications in transplantation and cancer immunotherapy.