Our study explored the effect of dietary lipids on growth and immunity and structure (head kidney, spleen and skin) of young grass, carp (Ctenopharyngodon idella). A total of 540 young grass carp with an average initial weight of 261.41 +/- 0.53 g were fed diets containing six graded levels of lipids at 5.9 -80.1 WIT diet for 8 weeks. After that, a challenge trial was conducted by injection of Aeromonas hydrophila over 2 weeks. The results indicated that compared with optimal lipids supplementation, low and excess levels of lipids down-regulated the mRNA levels of antimicrobial peptides, anti-inflammatory cytokines, inhibitor of kappa B alpha (I kappa B alpha) and ribosomal p70S6 kinase (S6K1), and up-regulated pro-inflammatory cytokines, nuclear factor kappa B p65 (NF-kappa B p65), NF-kappa B c-Rel (not p52), I kappa B kinase alpha (IKK alpha), IKK beta, IKK gamma, and eIF4E-binding protein (4EBP) mRNA levels in the head kidney and spleen of young grass carp (P < 0.05). Low or excess levels of lipids also increased reactive oxygen species (ROS) production and malondialdehyde (MDA) and protein carbonyl (PC) contents, reduced the activities of antioxidant enzymes (P < 0.05), down-regulate the relative mRNA levels of antioxidant enzymes and NF-E2-related factor 2 (Nrf2), and up-regulated the expression levels of Kelch-like ECH-associating protein la (Keap1a) and Keap1b in the head kidney and spleen. In addition, low or excess levels of lipids down regulated the mRNA levels of B-cell lymphoma-2 (Bcl-2) and inhibitor of apoptosis protein (IAP) in the head kidney and spleen, whereas up-regulated the mRNA levels of apoptotic protease activating factor-1 (Apaf-1), caspase 3, 7, 8 and 9 mRNA levels in the head kidney and spleen and Fas ligand (FasL) mRNA levels in the spleen of young grass carp, suggesting that low or excess levels of lipids could decrease the head kidney and spleen immune function, induce oxidative damage and apoptosis and impair antioxidant system of young grass carp. At last, low or excess levels of lipids also impaired the immune function and structure in the skin of young grass carp. Based on the quadratic regression analysis for PWG, skin haemorrhage and lesions morbidity and IgM content, the dietary lipids requirements for young grass carp were estimated to be 43.7, 60.2, 55.0 and 52.1 g/kg diet, respectively. (C) 2016 Published by Elsevier Ltd.