The climate of the Philippines is characterized by a large volume and seasonality of rainfall (Coronas 1920; Flores and Balagot 1969). The seasonality of rainfall is more extreme than that of temperature and is influenced mainly by the Asian monsoon, tropical cyclones, and the orographic effect. Cayanan et al. (2011) argued that heavy rainfall in western Luzon is induced by both the location and the track of the tropical cyclones during June–September and the orographic effect. Kubota and Wang (2009) estimated that the tropical-cyclone-induced rainfall ratio from July to October exceeds 50% in northern Luzon. Moreover, the mean seasonal march of rainfall amount clearly differs between the western and eastern coastal regions of the Philippines because of the Asian monsoon and the orographic effect, as shown by station rainfall data reported by Akasaka et al. (2007) and by the satellite data by Chang et al. (2005). Both the rainfall amount and the seasonality of rainfall are important factors for the agriculture and economics of the Philippines. This is because the seasonal march of rainfall is characterized by not only the seasonal rainfall amount but also the onset, peak, withdrawal periods, and duration of the rainy season, which influence planting decisions and the timing of harvesting in the cultivation calendar. Therefore, whether the seasonal march of rainfall will change with future global warming is of great concern to the Philippines. To answer this question with a prognostic analysis, we must first investigate the past interannual and long-term variability in the seasonal march of rainfall and its causes. Recent studies have explored interannual variability of the onset of the summer rainy season or the summer monsoon in or around the Philippines since the late twentieth century. Moron et al. (2009) used rainfall data collected at 76 stations between 1977 and 2004 and data from the Climate Prediction Center Merged Analysis of Precipitation (CMAP) from 1979 to 2005 to study the spatial and temporal variability of the onset of the summer monsoon and to assess the seasonal predictability of local onset dates. They identified important climatic factors that are essential for the prediction of the onset: the sea surface temperature (SST) over the tropical Pacific and Indian Oceans in March and the wind field at the 850 hPa level in May. Akasaka (2010) studied the interannual variability of the onset of the summer rainy season throughout the Philippines using daily rainfall data collected at 39 stations between 1961 and 2000. The author pointed out that the onset was consistently delayed after the mid-1970s and that longterm changes in the onset might be related to the timing of the shift in the location of the western edge of the subtropical high over the Philippines. Kajikawa and Wang (2012) studied interdecadal changes in the South China Sea (SCS) summer monsoon onset for the period of 1979–۲۰۰۸٫ They pointed out that the relatively early onset of the SCS appeared during 1994–۲۰۰۸ and that the onset processes were closely connected to the northward seasonal march of the intertropical convergence zone, the enhanced intraseasonal variability, and the number of tropical cyclones over the western North Pacific (WNP) and SCS. However, the interannual variability in the seasonal march of rainfall, including not only the onset but also the withdrawal and duration of the rainy season, has been scarcely discussed in earlier studies. The purpose of this study is to elucidate the interannual variability in the seasonal march patterns of rainfall in the Philippines since the late twentieth century. This study also aims to reveal the relationship between interannual variations in the seasonal march of rainfall and the low-level atmospheric circulation around the Philippines.