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Modification of liposomes using polyethylene glycol (PEG) results in steric hindrance to the phagocyte system and prolongation of blood circulation time. However, PEGylation can reduce radiolabeling efficiency (RE) when using the glutathione method for radiolabeling the liposomes. Therefore, we investigated the effect of the extent of PEGylation (PEG extent (PEGExt): 0, 5, 9.6, and 13.7 mol%) on the in vivo biodistribution of liposomes in Wistar rats, and RE with technetium-(99m) ((99m)Tc). PEGylated liposomes were prepared with egg phosphatidylcholine (egg PC, 1.85 mol%), cholesterol (1.0 mol%), and distearoylphosphatidylethanolamine-N-[polyethylene glycol] (DSPE-PEG; 0, 5, 9.6, and 13.7 mol%, respectively). The size distribution of the PEGylated liposomes was analyzed by a dynamic light scattering. The (99m)Tc-hexamethylpropylene-amine oxime ((99m)Tc-HMPAO) complexes were used for radiolabeling of preformed liposomes. The labeling efficiency and stability was analyzed with Sephadex G-15 column, and the biodistribution studies of (99m)Tc-liposomes after intravenous (i.v.) injection were also investigated with Wistar rats. The sizes of PEGylated liposomes decreased by increasing the PEGExt to 9.6 mol%, whereas sizes increased at 13.7 mol%. RE of (99m)Tc were greater than 90% for all PEGExt tested, and radiolabeling stability in human plasma was enhanced as a function of PEGExt. Liposomes without PEG were cleared rapidly from the blood and accumulated preferentially in the liver and the spleen. When PEGExt was increased, the accumulation in the organs decreased. This accumulation of PEG was maximized at 9.6 mol%. Accumulation of the liposomes in the spleen was increased again when PEGExt increased to 13.7 mol%. The splenic uptake of liposomes seemed to be dependent not only on PEGExt but also on the size of the liposomes. In conclusion, the PEG chains on the surface of liposome have no influence on the labeling efficiency, and the prolongation of circulation time was maximized at the 9.6 mol% of PEGylation.