T helper cell (TH) function, as assessed by interleukin-2 (IL-2) production and [3H]thymidine incorporation, was studied in 47 newly diagnosed untreated patients with Hodgkin's disease (HD) and 34 healthy controls. Three different stimuli were used to stimulate in vitro peripheral blood mononuclear cells (PBMC): influenza A vaccine (FLU), HLA alloantigens (ALLO) and phytohaemagglutinin (PHA). Four different patterns of TH function were observed in HD patients: (1) IL-2 production in response to all of the stimuli (40%); (2) IL-2 production in response to ALLO and PHA but not to FLU (26%); (3) IL-2 production in response to PHA alone (19%); and (4) failure to respond by IL-2 production to any of the three of the stimuli (15%). Thus, defective in vitro TH function was detected in the majority of these patients (60%). Defective TH function was observed in none of the 34 controls. Severely compromised TH function (patterns 3 and 4) tended to be associated with more advanced clinical presentation and more compromised haematological parameters (P < 0.05). The IL-2 production assay was more sensitive than the proliferative assay as only 30% of the HD patients failed to proliferate in response to FLU, and none failed to proliferate in response to either ALLO or PHA; this assay can detect subtle, multiple patterns of immune dysregulation in untreated HD patients. Our results suggest that HD is associated with a fundamental dysregulation in TH function, illustrate the complexity of such dysregulation, and raise the possibility that HD progression will be associated with a type-1-type-2 switch in immunoregulatory cytokine production.