In the last decade, we have observed an increased public and scientific interest in the clinical applications of medical cannabis. Currently, the application of cannabinoids in cancer patients is mainly due to their analgesic and anti-emetic effects. The direct effects of phyto-cannabinoids on cancer cells are under intensive research, and the data remain somewhat inconsistent. Although anti-proliferative properties were observed in vitro, conclusive data from animal models and clinical trials are lacking. Since immunotherapy of malignant diseases and bone marrow transplantation are integral approaches in hemato-oncology, the immuno-modulatory characteristic of cannabinoids is a fundamental aspect for consideration. The effect of cannabinoids on the immune system is presently under investigation, and some evidence for its immuno-regulatory properties has been shown. In addition, the interaction of cannabinoids and classical cytotoxic agents is a subject for further investigation. Here we discuss the current knowledge of cannabinoid-based treatments in preclinical models and the limited data in oncological patients. Particularly, we address the possible contradiction between the direct anti-tumor and the immune-modulatory effects of cannabinoids. Better understanding of the mechanism of cannabinoids influence is essential to design therapies that will allow cannabinoids to be incorporated into the clinic.