In 1981, an epidemic occurred in Spain, toxic oil syndrome (TOS), in people who consumed rapeseed oil denatured with 2% aniline, and it was one of the largest intoxication epidemics ever recorded. In 1989, a similar disease, eosinophilia-myalgia syndrome (EMS) was reported in the USA and was associated with the ingestion of L-tryptophan. The pathologic findings in TOS showed primary endothelial injury, with cell proliferation and perivascular inflammatory infiltrates. Immunologic mechanisms have presumably been operative in the pathogenesis and perpetuation of TOS. Our previous findings pointed to a T-cell activation during acute phase of the disease. In order to analyze which T-cell subset is involved on TOS, we have developed an mRNA extraction procedure from paraffin-embedded lung tissues in patients with pulmonary involvement. We analyzed mRNA expression from different cytokines (IL-1, IL-2, IL-4, IL-5, IFN-gamma, GM-CSF) and CD25 (interleukin 2 receptor) and CD23 (low affinity IgE receptor), using RT-PCR technique. In lung tissues from these patients a T-cell activation was observed. We found a significant increase in Th1 (P = 0.006) and Th2 (P = 0.003) cytokine profile in TOS patients with respect to controls. The increment in TH2 response with respect to TH1 is significant (P = 0.03) in TOS lung specimens. Non-significant differences were obtained in other cytokines and receptors studied as IL-1, CD25, CD23 and GM-CSF. Data presented in this paper are the first clear evidence that an immunological mechanism is directly implicated in this illness.