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.