Recent theories have established that, during an ongoing immune response, the lymphokines produced by TH1 and TH2 subsets of CD4⁺ T cells are critical to the effectiveness of that response. In vivo and in vitro studies have demonstrated that the type of environmental cytokines plays a determinant role in directing the development of naive T cells into TH1 or TH2 effector cells. Disregulated expansion of one or other subset may contribute to the development of certain diseases. To establish whether a similar situation might exist in the cells of the peripheral blood (PBMC) of colorectal cancer patients, we have performed immunological studies on a group of patients and a group of healthy subjects. We examined the interleukin-2 (IL-2), interferon γ (IFNγ), IL-4, IL-6 and tumour necrosis factor α levels in serum; the production of IL-4 and IL-2, with and without activating agents, by PBMC, tumour-draining lymph node lymphocytes and tumour cells; and the proliferative response of PBMC to IL-2, IL-4 and anti-CD3 monoclonal antibody (anti-CD3), which were variously combined. The data of the present study lead us to hypothesize that, because of suppressive effects probably due to environmental IL-4, in the peripheral blood of patients there seems to be a disregulation in the functionality of TH1 and TH2 subsets of CD4⁺ T cells, with an expansion in TH2 and a malfunction in TH1 cells. Moreover it seems that this disregulation increases with as the disease progresses through the stages, suggesting that it can be directly implicated in the mechanisms that allow the tumour to locate and progress in the host.