Pig (Sus scrofa domesticus) slurry (PS) is often applied year after year to the same fields, and its residual effects under Mediterranean conditions are not well understood. This study assessed the residual effects of PS combined with inorganic N fertilizer, on irrigated monoculture maize (Zea mays L.) yields and nitrate leaching. Grain yield and nitrate leaching in the fifth year were compared to those obtained with several N fertilizer rates (0, 180, 240, and 300 kg N ha−1). Previous PS applications resulted in N fertilizer replacement values that ranged from 226 to 306 kg N ha−1. The N budget in the fifth year indicated that there had been additional uncontrolled mineral N inputs in the PS treatments in comparison to mineral N treatments. These inputs were presumed to be associated with the residual effect of PS treatments applied during the previous 4 yr. In Mediterranean environments cumulative mineralization of repeated PS applications should be considered when making fertilizer recommendations. Nitrate leaching did not differ between the treatment with the lowest PS rate (30 Mg ha−1) and the fertilization rate of 300 kg N ha−1, while higher PS rates increased nitrate leaching. Pig slurry rates above 30 to 40 Mg ha−1 yr−1 are not environmentally sustainable in Mediterranean agricultural systems.