Abstract Polyethylene glycol (PEG) surface modification can make nanomaterials highly hydrophilic, reducing their sequestration in the reticuloendothelial system. In this study, polyamidoamine (PAMAM) dendrimers bearing gadolinium (Gd) chelates were PEGylated with different PEG-chain lengths, and the effects on paramagnetic and pharmacokinetic properties were evaluated. Specifically, Gd chelate-bearing PAMAM dendrimers (generations 4 and 5; G4 and G5) were conjugated with two different PEG chains (2 kDa and 5 kDa; 2k and 5k). Long PEG chains (5k) on the smaller (G4) dendrimer resulted in reduced relaxivity compared to non-PEGylated dendrimers, whereas short PEG chains (2k) on a larger (G5) dendrimer produced relaxivities comparable to non-PEGylated G4 dendrimers. The relaxivity of all PEGylated or lysine-conjugated dendrimers increased at higher temperature, whereas that of intact G4 Gd-PAMAM dendrimer decreased. All PEGylated dendrimers had minimal liver and kidney uptake and remained in circulation for at least 1 hour. Thus, surface-PEGylated Gd-PAMAM dendrimers showed decreased plasma clearance and prolonged retention in the blood pool. Shorter PEG, higher generation conjugates led to higher relaxivity. From the Clinical Editor In this study, polyamidoamine dendrimers bearing gadolinium (Gd) chelates were PEGylated with different PEG-chain lengths, and the effects on paramagnetic and pharmacokinetic properties were evaluated.