Hydrological regionalization is an important tool for the analysis of the spatial pattern of variations in hydrological phenomena. Regionalization means areal classification, i.e. the ability to attach to some location a label or number which is hydrologically meaningful. With the use of statistical methods, hydrological regions are analogous to areal classes. In this present study, pairwise grouping as well as principal components are applied to Swedish river runoff data in order to analyse the patterns of variation and to delimit regions where hydrological behaviour may be regarded as uniform. Pairwise grouping is the appropriate method to use on a national scale with heterogeneous hydrological regimes. Principal components (empirical orthogonal functions) demand homogeneity but within such regions describe very well the trends and variations.