In the Precambrian basement of SW Sweden two formations of granitic rocks have been strongly recrystallized but have preserved their granitic texture. Most magmatic crystals have been replaced by green-schist-facies mineral assemblages. δ18O whole-rock values for the granitic rocks vary from +1.7‰ to +8.2‰ (relative to SMOW) and extend below the normal range for granitic rocks (+6 to +10‰). Quartz-K-feldspar pairs show 18O16O fractionations of +5.4‰ and +7.8‰, indicating disturbance of magmatic equilibrium fractionations. It is concluded that the O-isotope redistribution took place during the pervasive recrystallization of the rocks, under the influence of a large-scale flux of water (partly) of meteoric origin, probably in a convective hydrothermal system set up by the intrusion of the granites themselves. Estimation of pre-alteration δ18O whole-rock values for one of the granitic units suggests a partly S-type origin.