More than 10% of Peruvian sugarcane is irrigated with ‘filter cake water’ (FCW). No responses to applied N have been reported on these soils. This study evaluated N gains and losses in sugarcane crops (H32-8560 cultivar) irrigated with FCW. Input of N via irrigation water corresponded to 101 kg N/ha. The inorganic N (representing 1% of total N) initially present in the soil prior to planting corresponded to 161 kg N/ha and the mineralized N from total N which becomes available during the growing season to 161 kg N/ha. The N by the aerial part of the plant was 272 kg N/ha; 75% of this amount was removed during harvest (204 kg N/ha), and the remainder (25%) was lost by pre-harvest burning (68 kg N/ha). Nitrogen within the stubbles, rhizomes and roots corresponded to 92 kg N/ha. The total input of N into the soil was estimated to be 423 kg N/ha and extractable N to 364 kg/N ha. The balance was in favor of N gains, with an average gain of 59 kg N/ha. This N-balance explains why results from numerous field experiments have demonstrated that there is no response to N fertilization in fields irrigated with FCW.