A group of earlier investigations has identified some specific regions over the world in which hydrometeorological surface parameters (i.e. precipitation and streamflow) appear to have consistent relationships with the extreme phases of the Southern Oscillation (SO). This paper examines a climatic link between the monthly streamflows in Turkey and the tropical El Nino and La Nina events using a large data set and comprehensive empirical methodology. Coherent and significant streamflow responses were detected in two core regions, namely the northwestern Anatolia (NWA) and the eastern Anatolia (EA). The areal extent of both regions with a defined seasonal signal for the case of each tropical event was determined. Some subregions whose indications are considered to be better than those for the whole core region were also isolated within the core region. In the Susurluk subregion identified in the NWA, the April-October seasonal positive streamflow anomalies were found to have a highly significant relation with the El Nino events. Although there is a possibility that the NWA region is influenced by the El Nino events, this was not confirmed by the hypergeometric test used to assign a significance level for the relationship. A similar analysis has shown that the La Nina events have no noticeable influences on streamflows in the NWA. For the EA region, above normal conditions have been observed during the April(0)-November(0) period and the May(0)-February(+) period for the El Nino and La Nina events, respectively. Moreover, the annual cycle analysis somewhat implies a modulation of regional streamflow in the core regions during the signal period. In some parts of the core regions, correlation results confirm these signals. The results of this study are discussed and are said to be in agreement with the most relevant previous studies concerning precipitation to understand complex global circulation dynamics. In conclusion, mid-latitude streamflow responses to the extreme phases of SO are detectable over Turkey. Copyright (C) 2001 Royal Meteorological Society.