The aim of this study was to investigate the changes involved in the evolution of nine cases of recurrent B-cell lymphomas. Using the polymerase chain reaction (PCR) on formalin-fixed, paraffin-embedded tissue from both the primary and the recurrent lymphoma of each case, monoclonality was demonstrated in every tumour. In all nine cases, the recurrent lymphoma was shown to belong to the same clone as the primary lymphoma. Eight of these cases were then investigated by immunohistochemistry for changes in Bcl-2 and p53 expression. Five out of eight of the primary lymphomas showed Bcl-2 overexpression. Two of the three cases initially negative for Bcl-2 expression became positive in the recurrence. One out of eight of the primary lymphomas was positive for p53 expression. Of the seven negative cases, one became positive for p53 expression in the recurrence. Both of the p53-positive cases showed high-grade histology. This study shows that Bcl-2 overexpression is probably an important early event in the development of B-cell lymphomas, although it may occur as a post-neoplastic event. p53 mutation is probably more important as a late event and may be associated with high-grade transformation.