Objective: In order to exclude the possibility that mistletoe therapy may result in immunosuppression, as indicated by a significant reduction of defined lymphocyte subsets, Patients and Methods:peripheral blood cells of 23 tumour patients were treated subcutaneously with increasing concentrations of aqueous mistletoe extracts (Helixor®). Results and Conclusions:Within an observation period of 7 months, the relative amount of lymphocytes and the number of natural killer (NK) cells increased while the number of lymphocyte subsets (i. e. CD19+ B cells, CD4+ T helper cells, CD8+ CD28– suppressor cells, CD8+ CD28+ cytotoxic cells) and the proportion of CD25+ (activated) cells within T cells showed a statistically remarkable trend; due to the multiple test problem of statistical evaluation this trend is not allowed to be termed significant. The leucocytes decreased insignificantly within the observation period. However, we were unable to verify a suggested increase of defined lymphocyte subsets within 2–3 months after the onset of mistletoe treatment. Nevertheless, for the parameters CD19+ B cells, CD4+ T helper cells, CD8+ cells, CD8+ CD28+ cytotoxic cells and CD16+/CD56+ NK cells we observed statistically remarkable peaks within die 2nd and 3rd month of therapy, confirming the hypothesis. The responses to the extracts were obviously interindividually different; the immune responses especially of patients with a lower number of peripheral T cells were less significant as compared to those of patients with adequate T cell numbers. Surprisingly, even an increase of the drug concentration >3 ng mistletoe lectin (as determined within the whole plant extract) per kg body weight enhanced the number of CD4+ T helper cells. A decreased immunological reaction on mistletoe extracts was shown especially for patients with a reduced number of peripheral T cells, whereas patients with normal T-cell number were more reactive.