Narcolepsy is a sleep disorder in which multiple factors, including environmental and genetic factors, are involved. A genetic factor strongly associated with the disorder has been found in the human leukocyte antigen (HLA) class II region: the haplotype, DRB1*1501–DQB1*0602, predisposes to narcolepsy. No susceptibility genes other than the HLA‐haplotype have been found. In this paper, we performed an association study of the tumor necrosis factor‐alpha (TNF‐α) gene located in the HLA class III region with human narcolepsy, in which we examined the known single‐nucleotide polymorphisms (SNPs) in the promoter region in 49 narcoleptic patients, who were all positive for DRB1*1501, and 111 healthy control individuals. The results indicated that the frequency of the genotype at position −857 (−857SNP) was significantly different between the patients and controls, and the allele frequencies of –857SNP revealed that the frequency of –857T was significantly increased in the patients as compared with that in the controls (P=0.0068). In addition, haplotypes presumed from HLA‐DRB1, –857SNP and HLA‐B loci suggested that –857T was mainly associated with DRB1 alleles other than DRB1*1501: the significant increase in frequency of –857T in the patients was not caused by allelic association with DRB1*1501. Therefore, it is conceivable that the TNF‐α with –857T was associated with narcolepsy independently of the strong association of DRB1*1501 with the disorder. Altogether, the data presented here lead us to propose that TNF‐α could be a new susceptibility gene in human narcolepsy.