This work analyzed precipitation data for the Ivaí river basin, Brazil, using daily, monthly and annual data for the period 1974 - 2001. These data were obtained from the Parana State's Office of Hydrological Resources. The basin is located in Parana State, between -22°54'S and -25°33'S, and -50°44'W and -52°42'W, comprising an area of 36 522 km2 and a length of 680 km approximately. The Ivaí river comprises the portion located at the union of the Patos and Sao Joäo rivers. Several cities have developed along this basin, including Maringá, Apucarana and Campo Mouräo. Precipitation data recorded throughout the year were analyzed, and the most significant data were chosen for analysis (1982, 1983 and 1985) through tracing isolines. From these, it became evident that in 1983 (the year when El Niño reached a significant importance) rainy-day isolines were considerable above those for any other year of analysis. Conversely, 1985 (the year when La Niña occurred) showed isolines with a few rainy days in the period of study. Within the basin, precipitation is influenced by year-to-year variability (El Niño or La Niña). It is worth mentioning that, for each event, there is a marked positive (El Niño) or negative (La Niña) anomaly, the significance of which varies according to the event.