Dynamic light scattering measurements have been made on two series of polymacromonomer samples consisting of polystyrene with 33 styrene side-chain residues (F33) in cyclohexane at 34.5 °C (a theta solvent) and with 65 styrene side-chain residues (F65) in cyclohexane at 34.5 °C and toluene at 15 °C (a good solvent) to determine the translational diffusion coefficient D as functions of the weight-average degree of polymerization of the main chain, Nw, ranging from 15 to 3030 for F33 and from 42 to 1250 for F65. The D data are compared with those previously obtained for a polystyrene polymacromonomer with a shorter side chain length of 15 styrene residues. The hydrodynamic radius RH at fixed Nw is larger for a longer side-chain polymer and in toluene than in cyclohexane (for F65), reflecting the higher backbone stiffness for the longer side chain and in the good solvent. The Nw-dependence of RH and that of the reduced hydrodynamic radius (the ratio of RH to the radius of gyration ‹S2›1/2) for the respective polymacromonomers are fitted by the theoretical curves for the wormlike chain with the model parameters leading to fits of previous data for the intrinsic viscosity ([η]) and ‹S2›, provided that the chain diameters d are taken to be 1.29–1.35 times those from [η]. This discrepancy in d is similar to what was found for thin stiff chains, revealing certain shortcomings of the current hydrodynamic theories. It is pointed out that polystyrene polymacromonomers with large diameters relative to Kuhn’s segment length are hydrodynamically similar to flexible chains for which the available theories for D contain an error of about 15% in the Gaussian-chain limit.