Rainfall-runoff relationships are widely used in many engineering hydrologic designs in urban and rural areas. Such relationships are obtained through the application of regression analysis in many studies. Unfortunately, in the classical regression approach to determine rainfall-runoff relationships, internal uncertainties are not taken explicitly into consideration. In this paper, an alternative to the classical regression approach is proposed through fuzzy system modeling. It is concluded that the fuzzy systems approach yields comparatively less relative error than a regression approach and, therefore, it is recommended for use in future. The application is presented for rainfall-runoff records at two sites near Istanbul, Turkey.