Objective.To assess the effects on bone mineral density (BMD) of prolonged anti-tumor necrosis factor (anti-TNF) therapy in patients with spondyloarthritis (SpA); to compare the BMD changes to those observed in SpA patients not treated with anti-TNF; and to identify the predictors of these changes.Methods.Fifty-nine patients with SpA according to the European Spondylarthropathy Study Group criteria who were treated with anti-TNF therapy for at least 4 years were included. Thirty-four patients with SpA from an international longitudinal observational study (OASIS cohort) were used as a control group. Lumbar spine and hip BMD were measured by dual-energy x-ray absorptiometry at baseline, after 1 year, and after at least 4 years.Results.Over an average 6.5 years’ followup, the increase in BMD was 11.8% (± 12.8%) at the lumbar spine (p < 0.0001) and 3.6% (± 9.3%) at the great trochanter (p = 0.0001) in patients treated with anti-TNF. At the lumbar spine, the increase was similar in patients with and those without syndesmophytes. BMD changes were significantly higher in the anti-TNF group than in the control group at lumbar spine (p < 0.0001), at femoral neck (p = 0.002), and at trochanter (p = 0.011), but not at total hip (p = 0.062). Multivariate analysis showed that the predictors of lumbar spine BMD changes in the total population were the use of anti-TNF (p < 0.0001) and, in the anti-TNF therapy group, the 1-year lumbar spine BMD change (p = 0.007).Conclusion.This study shows that prolonged anti-TNF therapy increases lumbar spine and trochanter BMD. This effect should be taken into account before introducing antiosteoporotic treatment in these patients.