Many examples of oligoclonal T-cell expansion in infiltrated diseased tissues have been reported. However, it remains to be established whether such observations can be generalized and to what extent oligoclonal patterns obtained after in vitro culture of T-cell infiltrates reflect in vivo situations. Using new high resolution analysis which requires no in vitro cellular expansion, we detected such oligoclonal T-cell expansions in 7/7 melanoma tumour biopsies, 3/3 biopsies of inflammatory skin during acute graft versus host disease (aGVHD) after allogeneic bone marrow transplantation (alloBMT) and 7/7 synovial membranes from patients with rheumatoid arthritis. Thus, oligoclonal T-cell expansions are readily observed when a sufficiently sensitive detection method is used, suggesting that similar expansions are the rule among T-cell infiltrates in different diseases. This observation and the monitoring of the in vivo evolution of such expansion during the course of the disease and during in vitro culture should have important clinical implications.