We demonstrate the conjugation of the cancer drug doxorubicin (DOX) to poly(methacryloyloxyethyl phosphorylcholine) (polyMPC), linked by hydrazone groups, using (1) a one-pot ATRP/click sequence, and (2) a post-polymerization conjugation strategy. While the one-pot method gave polyMPC-DOX conjugates in a facile single step, post-polymerization conjugation gave higher-molecular-weight polymers with very high DOX loadings. DOX release from the polyMPC backbone was pH-dependent (faster at pH 5.0 than at pH 7.4) owing to the hydrazone linkage. Half-life values of DOX release ranged from 2 to 40 h at pH 5.0. Cell culture experiments showed that highly loaded polyMPC-DOX conjugates exhibited higher intracellular drug accumulation and lower half-maximal inhibitory concentration (IC(50)) values, while a polymer with 30 wt % drug loading showed a maximum tolerated dose in the range of 30-50 mg/kg DOX equivalent weight in healthy mice.