Ceramides mediate anti proliferative responses, and it has been proposed that increasing the level of ceramides in cancer cells may have a therapeutic antitumor effect. However, ceramides, because of their high "packing parameter" (PP), do not form lipid assemblies that can be dispersed in a form suitable for intravenous administration. We found that nanoliposomes containing short- or medium-chain ceramides are unstable because of their very high (> 1.3) PP. To overcome this major obstacle, we included the lipopolymer (2k)PEG-DSPE, which reduces the additive PP. The presence of PEG-DSPE allows the formation of highly stable (> 1 year) ceramide (Cer)-containing nanoliposomes suitable for systemic administration. Using tumor cell lines, we found that the ceramide cytotoxicity was not impaired by their inclusion in nanoliposomes. The use of C-14-labeled ceramides shows that the C(6)Cer, but not C(16)Cer, was transferred from the nanoliposomes to the cells and metabolized efficiently. The difference between the two ceramides is related to the large difference between their critical aggregation concentration and was correlated with the much higher cytotoxity of liposomal C(6)Cer. The activity of (2k)PEG-DSPE as a steric stabilizer (as previously shown for Doxil) was also confirmed for C(6)Cer-containing nanoliposomes. The (2k)PEG-DSPE lipopolymer significantly reduced the desorption rate of the ceramide from the liposome bilayer, thereby allowing liposomes containing C(6)Cer to reach the tumor site and to demonstrate therapeutic efficacy.