Although T cells are often a prominent component of follicular (+/- diffuse) centroblastic/centrocytic (cleaved follicular centre cell--FCC) lymphomas (CB/cc/F +/- D), their evolution during the course of the disease and prognostic significance are uncertain. A total of 127 histopathologic and immunophenotypic studies from 77 patients with at least one histologic diagnosis of CB/cc/F +/- D were therefore retrospectively reviewed to address these questions. The proportion of T cells in each biopsy was analysed with reference to both histologic (diagnosis--Kiel and Lukes/Collins classification, presence of transformation, growth pattern, presence of residual mantle zones, biopsy sites) and clinical (biopsy date and number, treatment status) parameters. The actuarial survival for those patients whose first or second biopsy showed CB/cc/F +/- D and had immunophenotypic studies was also compared based on the proportions of T cells present. Among the biopsies which were obtained from 0 to 153 months following diagnosis, the mean proportion of T cells was 25 +/- 18 per cent. The only histopathologic group that showed a significantly different proportion of T cells were lymphomas of large cleaved FCC type (38 +/- 20 per cent). Although the number of T cells present varied greatly between different patients and among different biopsies in the same patients, neither biopsy number nor treatment status affected the mean proportion of T cells in CB/cc/F +/- D. With the exception of those biopsies from 1 to 11 months following diagnosis, the interval time between diagnosis and biopsy also did not affect the number of T cells present. Finally, the proportion of T cells in the lymphomas could not be shown to be of prognostic importance. Thus, the proportion of T cells in de novo CB/cc/F +/- D, at relapse or in transformation does not vary in a consistent fashion. Although extremely variable, the number of T cells present cannot be considered a feature of earlier or later disease and is not of demonstrable prognostic importance. Studies of specific subsets of T cells may provide more definite associations.