The association between the histocompatibility antigen HLA-B27 and the seronegative spondylarthritides such as ankylosing spondylitis and Reiter's syndrome is dramatic. A question that arises in practice is, when should a clinician request HLA-B27 typing in the assessment of a patient with a rheumatologic complaint? Generally, diagnosis of these spondylarthropathies depends on history, clinical and radiologic examination, and, occasionally, confirmatory laboratory tests. This paper reviews the criteria for the diagnosis of the spondylarthritides, discusses the sensitivity and specificity of HLA-B27 typing in these conditions, analyzes the relation between HLA-B27 status and prognosis, and defines the role of genetic counseling. It is concluded th,t knowledge of the patient's HLA-B27 status provides only minimal help to the physician. Indiscriminate typing is to be deprecated.