The influence of spherical aberrations on laser-induced plasma formation in water by 6-ns Nd:YAG laser pulses of 1064 nm wavelength was investigated. Experiments and numerical calculations were carried out for focusing angles similar to those used for intraocular microsurgery. Wave form distortions of 5.5 lambda and 18.5 lambda between the optical axis and the 1/e2 irradiance values of the laser beam were introduced by replacing laser achromats in the delivery system by plano- convex lenses. Aberrations of 18.5 lambda led to an increase of the energy threshold by a factor of 8.5. The threshold irradiance calculated using the diffraction limited spot size was 10 times increased as compared to the case of minimized aberrations and 48 times larger than the actual threshold. The threshold calculated with the measured focus diameter was, on the other hand, reduced by a factor of 35. This reduction is due to the presence of hot spots in the focal region of the aberrated laser beam. In these hot spots, the threshold irradiance is probably unchanged, but the threshold value is reduced when averaged over the whole measured diameter of the beam waist. The determination of breakdown threshold in the presence of aberrations leads, hence, to strongly erroneous results. In the presence of aberrations, the plasmas are up to 3 times longer and the transmitted energy is 17 - 20 times larger than without aberrations. Aberrations can thus strongly compromise the precision and safety of intraocular microsurgery where they may arise through the use of inappropriate contact lenses, tilting of the lens, and oblique light passage through the ocular media. They can further account for a major part of the differences in breakdown threshold and plasma transmission values reported in previous investigations.