Iscador is being used by many patients as unconventional anticancer and immunomodulating therapy. To determine the toxicity profile and biochemical effects of Iscador Qu Spezial (Weleda AG Schwäbisch Gmünd, Germany) in human immunodeficiency virus (HIV)-positive patients and healthy controls, we performed a phase I/II study. Escalating doses of Iscador Qu Spezial, standardized for its lectin and viscotoxin content, were administered to 16 HIV-positive patients and 8 healthy subjects during a period of 6 to 8 months. Iscador Qu Spezial preparations were administered twice per week subcutaneously in increasing doses (ie, 0.01 mg, 0.1 mg, 1.0 mg, 2.0 mg, 5.0 mg, and 0.1 mg/kg for 2–6 weeks per dose). Drug-related adverse effects were flulike symptoms, gingivitis, fever, local erythema, and eosinophilia. These side effects were never severe. The incidence of systemic adverse events was highest in HIV-positive patients. Furthermore, increased urea levels and slightly decreased total protein caused by a minor decrease in albumin were observed. None of the HIV-positive patients progressed in disease stage. Iscador Qu Spezial can be administered safely to immunocompromised patients.