Tumour-infiltrating T lymphocytes (TIL-T) have been implicated in playing a role in controlling tumour growth. We evaluated TIL-T in 55 cases of de novo diffuse large B-cell lymphoma (DLBCL) using three- or four-colour flow cytometric immunophenotyping (FCI). The percentage of TIL-T varied from 3% to 72% of total viable cellular events (mean 32 ± 20%). The CD4:CD8 ratio varied from 0·17 to 13 (mean 2·3 ± 2·2). Cases with ≥ 20% T cells and those with CD4:CD8 ratios ≥ 2·0 showed a significantly better overall survival (P = 0·017 and P = 0·034 respectively). These findings were independent of clinical stage at diagnosis. The T-cell percentage and CD4:CD8 ratio were moderately correlated (Spearman correlation coefficient = 0·47, P = 0·001) and multivariate analysis revealed that the association of the two factors with prognosis was mutually dependent. The T cells in 23 cases were studied for CD45RO. The mean percentage of total T cells expressing CD45RO was 86 ± 10%. There was a trend towards better survival for those patients with a higher percentage of CD45RO+ T cells (P = 0·06). These results suggest that TIL-T, particularly CD4+ T cells, may play a role in the control of DLBCL, and measurement of T-cell percentage and T-cell subsets using FCI may be useful in predicting the clinical behaviour of DLBCL.