Liposomes encapuslating positron emitters are applicable for diagnostic imaging and are useful to investigate the real-time liposomal trafficking in vivo. Long-circulating liposomes encapsulaing [2-(18)F]-2-fluoro-2-deoxyglucose were administrated to tumor-bearing mice, and a PET scan was performed. Small-sized long-circulating liposomes (100 nm) tended to accumulate in tumor tissues of tumor-bearing mice as compared with conventional liposomes. Then the size effect on trafficking of long-circulating liposomes was investigated. Large-sized liposomes (>300 nm) accumulated in liver and spleen in a time dependent manner. On the contrary, small-sized ones (<200 nm) were transiently accumulated in the liver right after injection, but the accumulation decreased time dependently, suggesting that, although the majority of small long-circulating liposomes remain in bloodstream, some extravasate once into interstitial spaces in liver which re-enter into bloodstream again. Next the trafficking of so-called long-circulating liposomes, i.e., liposomes modified with ganglioside GM1, palmityl glucuronide (PGlcUA), and polyethylene glycol (PEG), in tumor-bearing mice was examined. The accumulation of all three kinds of long-circulating liposomes in liver decreased time-dependently, and PGlcUA-liposomes could avoid liver-trapping the most efficiently. Tumor accumulation of liposomes was obvious for PGlcUA-liposomes and PEG-liposomes from immediately after injection, but not for GM1-liposomes. Finally, the trafficking of differently charged liposomes was investigated in normal mice. The accumulation of positively charged liposomes containing 1,2-dimyristyloxypropyl-3-dimethyl-hydroxyethyl bromide was different from that of neutral and negatively charged DCP-liposomes. The agglutinability of and serum protein ginding to positively charged liposomes were marked, suggesting that these factors affect the high accumulation of DMRIE-liposomes in liver. Non-invasive PET analysis of liposomal trafficking is beneficial for obtaining information about liposomal drug delivery, and long-circulating liposomes might be useful for diagnostic tumor imaging by PET.