Inflammatory diseases frequently impair linear growth. Crohn's disease inhibits growth in up to one third of affected children. In rats with trinitrobenzenesulphonic acid-induced colitis, 40% of growth impairment is attributable to inflammation, with the rest being due to undernutrition. In transgenic mice without inflammation, raised IL-6 retards growth, suppressing insulin-like growth factor (IGF)-I. We hypothesized that IL-6, induced by intestinal inflammation, suppresses growth and inhibits IGF-I expression. Therefore, an anti-IL-6 Ab was given to rats with trinitrobenzenesulphonic acid colitis. The Ab did not improve nutrient intake or decrease inflammation compared with untreated disease controls, but it significantly restored linear growth (P ‫؍‬ 0.023) and increased IGF-I (P ‫؍‬ 0.05). In humans, the IL-6 ؊174 G͞C promoter polymorphism affects IL-6 transcription, with the GG genotype inducing the greatest IL-6 levels. Because IL-6 is increased in Crohn's disease, we further hypothesized that growth failure would vary with the IL-6 ؊174 genotype. At diagnosis, among 153 children with Crohn's disease, those with the IL-6 GG genotype were more growthretarded than those with the GC or CC genotypes (height SD score, ؊0.51 vs. ؊0.10; P ‫؍‬ 0.031). Also, the patients with the IL-6 GG genotype had higher circulating levels of C-reactive protein, an IL-6-induced product (36 vs. 18 mg͞dl, P ‫؍‬ 0.028). However, their risk of developing Crohn's disease was similar to other genotypes when compared with 351 healthy controls (P ‫؍‬ 0.7). Thus, the IL-6 ؊174 genotype mediates growth failure in children with Crohn's disease.Crohn's disease ͉ height ͉ insulin-like growth factor I ͉ C-reactive protein ͉ food intake