Recently, we demonstrated that loading of HAuCl(4) in poly(ethylene glycol) (PEG)-attached poly(amidoamine) (PAMAM) G4 dendrimers and subsequent reduction with NaBH(4) yield dendrimers encapsulating gold nanoparticles (Au NPs), which have photoinduced heat-generating properties. This study was undertaken to enhance photothermal properties of the Au NP-incorporated PEG-attached dendrimers by growing Au NPs in the dendrimers. Repeated loading of HAuCl(4) in the PEG-attached dendrimers and subsequent reduction with NaBH(4) enhanced the surface plasmon resonance, indicating that Au NPs were grown in the PEG-attached dendrimers using that procedure. Transmission electron microscopy (TEM) analysis revealed that the size of Au NPs formed in the dendrimers increased with the number of repetitions of HAuCl(4) loading and subsequent reduction in the dendrimers, although the size distribution of the Au NPs remained narrow. The photoinduced-heat generation capability of the Au NPs-encapsulating dendrimers increased as the Au NPs grew. These dendrimers with Au NPs exhibited strong cytotoxicity against HeLa cells under visible light irradiation. The result demonstrates that PEG-attached dendrimers encapsulating the grown Au NPs might be useful as devices for target-specific therapy when used with light irradiation.