Several studies have highlighted the importance of the microenvironment in the behaviour of follicular lymphoma (FL). We conducted an immunohistochemical analysis to assess the role of different cell subpopulations, i.e. CD8, CD68 and forkhead box protein P3 (FOXP3)-positive regulatory T cells (Treg cells) in 84 lymph nodes of 58 patients with FL (58 at diagnosis, 26 at relapse). Since the overall effect of Tregs is considered to depend on their number in relation to CD8(+) effector T cells, we determined the corresponding ratio for each patient and correlated the results with clinical parameters. The interfollicular CD8/FOXP3(+) cell ratio was significantly higher in patients with histological grade 3 tumours (2.04 vs. 1.63) and with a high risk FLIPI index (2.99 vs. 1.53) compared to those with grade 1-2 tumours or a low-intermediate FLIPI index. Similar results were obtained for the follicular CD8(+)/FOXP3(+) cell ratio. The interfollicular CD8/FOXP3 ratio was found to have prognostic value [a 5-year overall survival (OS) of 82 vs. 59% for a ratio of ±1.68]. In addition, an interfollicular FOXP3(+) cell number of more than 86 cells/mm(2) was correlated with a more favourable outcome (p=0.03). When patients at diagnosis and relapse were compared, a significant difference (p=0.05) in the localization (interfollicular vs. intrafollicular) of FOXP3(+) cells was observed. The CD8/FOXP3 ratio in the interfollicular areas was significantly different (1.66 at diagnosis vs. 2.2 at relapse, p=0.05). The presence of a small number of FOXP3(+) cells with a high CD8/FOXP3 ratio is probably the indicator of an active immune response during tumour development, with lymphoma cells acting as targets or bystanders.