Epidemiologically-based studies have shown that 20-50% of all cases of early arthritis cannot be classified. More recent data came from experiences with an outpatient clinic especially for early arthritis. Of 149 patients with inflammatory rheumatic disease, 39 (26%) were diagnosed as undifferentiated arthritis and 22 (15%) had a probable diagnosis. Therefore, despite diagnostic progress in recent years, only half of all cases can be diagnosed definitely. Follow-up studies of patients with HLA-B27 positive arthritis and undifferentiated spondylarthropathy show the difficulties of early diagnosis and the heterogeneity of disease course and prognosis. Early diagnostic criteria combining the result of the HLA-B27 test with history, symptoms, erythrocyte sedimentation rate and radiological spinal signs can predict or exclude the development of ankylosing spondylitis (AS) at an early stage of the disease in three-quarters of patients, but the diagnosis of AS should not be excluded before 5 to 10 years' observation.