Narcolepsy is a neurological disorder known to be tightly associated with HLA-DQA1*0102 and DQB1*0602. In this study, we have examined if homozygosity for DQB1*0602 increases disease susceptibility and/or severity. Patients diagnosed at Stanford University (n = 160) or enrolled in a multicenter clinical trial (n = 509) were included in this analysis. In both African-Americans and Caucasian-Americans with or without cataplexy, a significantly higher than expected number of subjects were DQB1*0602 homozygotes. Relative risks were 2-4 times higher in DQB1*0602 homozygotes vs heterozygotes across all patient groups. In contrast, symptom severity did not differ between DQB1*0602 homozygous and heterozygous subjects. These results indicate that HLA-DQB1*0602 homozygosity increases susceptibility to narcolepsy but does not appear to influence disease severity.