Cytogenetic, molecular genetic and clinical information was collected for 102 cases of diffuse large cell lymphoma (DLCL) ascertained in a diagnostic laboratory over a 3-year period. Nineteen cases showed evidence of either a t(14;18) or a rearrangement of one of three genomic probes for breakpoints at 18q21. Clinical and histologic evidence of transformation from follicular lymphoma or chronic lymphocytic leukaemia was available in six cases. Except for age, prognostic clinical variables (LDH, stage, extranodal involvement) were similar between the 18q21 rearranged patients and DLCL patients without 18q21 rearrangement. At a median follow-up in excess of 2 years for both groups, there was no difference in overall survival between the 18q21 rearranged group compared to DLCL patients lacking this genetic abnormality. The median disease-free survival for the 18q21 rearranged group, however, was significantly shorter and survival in partial remission longer. The propensity for extended survival of the 18q21 rearranged DLCL patients with residual or recurrent disease resembled the clinical behaviour of nodular lymphoma patients with t(14;18). These results suggest that cytogenic or molecular genetic identification of a chromosome 18q21 translocation may be of prognostic significance in the analysis of treatment protocols for patients with DLCL.