Tissue typing was performed on 14 narcoleptics as defined by both strict sleep laboratory and clinical criteria. Six of these patients were blacks from North America, a race underrepresented in previous studies. All patients were HLA-DR2-antigen positive and had the same HLA-DR2 subtype. Clinical severity of disease was not correlated with HLA-DR2 heterozygosity or (putative) homozygosity. This study confirms that the extremely high association between HLA-DR2 and narcolepsy holds across comparisons of the three races studied to date when both clinical and sleep laboratory data are used. The presence or absence of HLA-DR2 in patients presenting with hypersomnolence may help support or exclude, respectively, a diagnosis of idiopathic narcolepsy.