Mitogen-stimulated basal and maximal interleukin-2 production has been measured in 60 control subjects and 45 patients with gastrointestinal cancer (14 localized and 31 advanced). Peripheral blood T cell subsets in these subjects were also measured. In patients with advanced gastrointestinal cancer interleukin-2 production (mean +/- s.e.m. units/ml) is impaired when compared with that of control subjects (26.5 +/- 7 versus 61.1 +/- 9, P less than 0.0001) or patients with localized cancer (26.5 +/- 7 versus 59.4 +/- 13, P less than 0.02). This cannot be restored to normal by in vitro irradiation of the lymphocytes, suggesting that the impaired function is not due to IL-2 suppressor cells. Using monoclonal antibodies the percentages of T cell subsets were similar in all groups and we therefore conclude that the reduced production of IL-2 in these patients is due to deficient helper T cell function. These results provide a rational basis for the administration of exogenous IL-2 in the future management of patients with advanced gastrointestinal cancer.