TEMPO-based stable radicals were attached to dendrimers of variable size and used to control radical polymerization of styrene, vinyl acetate, and (meth)acrylates. Thermal polymerization of styrene with [G-2]-TEMPO proceeded in a similar but not better controlled manner than with TEMPO alone. In the polymerization of styrene initiated with BPO, the kinetics and the molecular weight/conversion relations showed the same tendency as with TEMPO, though the polydispersity was higher than in the absence of dendrimers. This indicates that homolytic cleavage occurs at the reaction temperature, monomer can diffuse inside the cavity of the dendrimer, and polymer is at least partially compatible with the dendrimer. Model reactions indicate that the higher observed polydispersities could not be ascribed to transfer to benzyl hydrogen atoms. Therefore, the higher polydispersities could originate from the self-initiated polymerization of styrene outside of the dendrimers as well as the slow exchange of these chains with chains attached to dendrimers. To evaluate the effect of self-initiation, vinyl acetate, methyl methacrylate, and n-butyl acrylate were used; however, the resulting polymers were incompatibile with the dendrimers. The growing chains are easily released from the cavity of the dendrimers into solution, and their return into the cavities of the dendrimers was not fast enough to control the polymerization. The results of the polymerization in decalin solution indicate that polymerization of these monomers is better controlled but only at the early stages of the polymerizations. The subsequent precipitation of polymers attached to the dendrimers prevents further polymerization. These results suggest that the main reason for the higher observed polydispersities is not termination between growing chains but either decomposition of alkoxyamines or self-initiation occurring simultaneously with the slow propagation.