The prevalence and pattern of ankylosing spondylitis (AS) can vary from country to country, according to genetic and environmental factors. This study aims to analyze the patterns of disease in a population of Iranian patients with AS. We performed a prospective study (2002-2007) analyzing 98 patients with diagnosis of AS according to the modified New York criteria. Selected patients underwent complete clinical (initial symptom, axial and peripheral involvement, heel enthesitis, extra-articular manifestations) and radiological (sacroiliac, lumbar, thoracic, and cervical spine) investigations, and these data were compared with sex, age at onset, and HLA-B27. There was predominance of men (71.4%), adult onset (>16 years, 90.8%), and positive HLA-B27 (73.4%). Family history of AS was noted in 14.3% of the patients. The predominant initial symptoms were inflammatory low back pain (44.2%). Radiological findings included syndesmophytes in 34.7% and "bamboo spine" in 16.3% of patients. Acute anterior uveitis was noted in 44.9% of patients. Male sex was associated with involvement of shoulder (P = 0.001). Female sex and juvenile-onset AS were associated with extra-articular involvement. Positive HLA-B27 was associated with hip involvement (P = 0.042) and adult-onset AS (P = 0.035). Analysis of the patterns of disease in this population of 98 southern Iranian patients with AS revealed that female sex and juvenile-onset AS were associated with extensive extra-axial involvement; and HLA-B27 was associated with hip involvement.