In this large study of well-characterised Danish patients with mild to severe asthma, we found that patients with the CHI3L1 (131 CC, rs4950928) genotype had higher serum concentrations of YKL-40 compared with patients with the CHI3L1 (−131 GG, rs4950928) genotype. The gene discussed is CHI3L1; the disease is asthma.