The electronic and geometrical structures of the ground and excited states of the 3d-metal dioxides ScO2, TiO2, VO2, CrO2, MnO2, FeO2, CoO2, NiO2, CuO2, and ZnO2 along with their singly charged negative ions have been calculated using the density functional theory with generalized gradient approximation for the exchange-correlation potential. We have considered ore, peroxo, and superoxo isomers in both neutral and anionic series. The ground states of all 3d-metal dioxides and their anions possess oxo forms, except for copper dioxide, which prefers a superoxo form. All the dioxides have a large number of isomers closely spaced in total energy. The champion, FeO2-, possesses five isomers within an energy range of 0.35 eV. The energy gap between ore, peroxo, and superoxo isomers decreases along the series from TiO2 to ZnO2. ScO2 has peroxo and superoxo isomers which are close in total energy to its oxo ground state. The electron affinities are computed for all types of isomers and are compared to available experimental data. On the average, our theoretical values are within 0.2 eV from the corresponding experimental data. All the ground-state dioxides and their anions are found to be thermodynamically stable except for ZnO2, which is unstable toward dissociation of molecular oxygen.