Traditionally, allergic contact dermatitis (ACD) has been associated with the activity of Th1 lymphocytes that secrete interferon gamma. Recent evidence indicates that other cells, e.g. interleukin 5 (IL-5)-secreting Th2 or Tc2 cells may be among the key effectors of ACD. The aim of the present study was to assess the influence of nickel-specific IFN-gamma secretion (marker of Th1 and Tc1 activity) and IL-5 secretion (Th2 and Tc2) on the clinical outcome (patch test score) in nickel-allergic patients. 40 women with suspicion of ACD were involved, aged from 14-54 (median 31.5) years. They were patch tested with NiSO₄. Peripheral blood mononuclear cells (PBMC) from the patients were cultured and analysed for IFN-gamma and IL-5 secretion in response to NiSO₄. A series of statistical models (classical logit or cloglog link function) were used. We demonstrate that nickel-specific IL-5 secretion by PBMC is correlated with the intensity of patch test reaction (p=0.05), with no significant effect of IFN-gamma. An increase in the nickel-specific IL-5 secretion from PBMC by 10 pg/ml is associated with a 10-20% increase (depending on statistical model) in the odds ratio of the patient to have a higher patch test score. These findings support the assumption that cells secreting IL-5 (e.g. Th2, Tc2) play a more important role in the pathogenesis of ACD than previously thought.