During the last decade, liposomes (artificial phospholipid vesicles) have been used with growing success as pharmaceutical nanocarriers of various drugs. Fields of application include lipid-based formulations to enhance the solubility of poorly soluble antitumor or antifungal pharmacological ingredients. The use of pegylated liposomes for passive targeting to solid tumors as well as vector-conjugated liposomal carriers for active targeting has been the major success. Such formulation and drug targeting strategies enhance the effectiveness of anticancer chemotherapy and strongly reduce the risk of toxic side-effects. Unfortunately liposomal drugs after intravenous activation may display a new kind of side effect the Complement Activation Related Pseudo Allergy (CARPA). In some very rare cases liposomal drug treatment induces complement activation and subsequent collapse of the pulmonary circulation. The CARPA reaction hindered the spreading of liposomal based therapies. However we have hopes that our research may result in new CARPA free liposomal formulations, predictive clinical tests for CARPA reactivity and simple premedication for the accepted liposomal drug treatments as Doxil or Ambisome.