Giant cell tumours of tendon sheath vary from solitary nodules to a multinodular variety that exhibits local infiltration. Recent advances in molecular oncology have defined a gene, nm23, expressed in normal cells which is responsible for inhibiting infiltration. The aim of this study was to investigate the expression of nm23 in a series of 52 giant cell tumours using immunohistochemistry and to assess its prognostic potential. nm23 expression was absent in 21%, of tumours and this was associated with a highly significant risk of local recurrence (P<0.0001). Multivariate analysis of outcome showed nm23 expression to be more reliable than other clinicopathological parameters for predicting outcome. This immunohistochemical test for nm23 is easily performed on standard paraffin sections and is recommended as an accurate prognostic marker for giant cell tumours of tendon sheath.