Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease. Decreased bone mineral density (BMD) is a common complication of AS, with a prevalence range of 19 to 62 %. Many studies have shown decreased BMD in AS with long disease duration, but only a few studies investigated BMD in early AS. The prevalence of decreased BMD in early disease stages of AS has not yet been clearly described, and for that reason, we reviewed the literature which describes the prevalence of decreased BMD in AS patients with a short disease duration (<10 years). In this review, we included articles which used the modified New York criteria for the diagnosis of AS, included patients with a disease duration of less than 10 years, and used the WHO criteria for osteopenia and osteoporosis. Decreased BMD was defined as a T score < -1.0, including both osteopenia and osteoporosis. For this review, only articles that acquired BMD data of lumbar spine and femoral neck by DXA were used. The literature search provided us 35 articles of which 7 matched all our criteria, and they will be further outlined in this review. The overall prevalence of decreased BMD of the articles reviewed is 54 % (n = 229/424) for lumbar spine and 51 % (n = 224/443) for femoral neck. The prevalence of osteopenia vs. osteoporosis for lumbar spine is 39 vs. 16 % and for femoral neck, 38 vs. 13 %. This review showed a high total prevalence of 51-54 % decreased BMD and 13-16 % osteoporosis in AS with a short disease duration. This high prevalence was not to be expected in a relatively young and predominantly male population. Further research is needed to determine the clinical relevance of this low BMD by investigating the relation between low BMD and vertebral and nonvertebral fractures at this early stage in AS.