Fourteen out of 283 patients with chronic lymphoid leukaemia, 2 out of 47 with Waldenström's disease and 16 out of 136 with low malignancy lymphoma (follicular with predominant small cleaved cells, mixed follicular with small and large cells, or diffuse lymphocytic) underwent histological transformation of their disease into a highly malignant lymphoma (immunoblastic in 14 of the 32 cases). There are no clinical or biological signs that predict these changes which seem to occur haphazardly in time. Seven patients died within one month of the diagnosis. The classical treatments failed. Complete remission was obtained in 5 cases with the intensive and sequential chemotherapy used for initially aggressive lymphomas.