OBJECTIVE Anemia is a common complication in patients with inflammatory diseases such as ankylosing spondylitis (AS). This post hoc analysis of a large, randomized, placebo-controlled trial examined the effect of infliximab on hemoglobin levels, physical function, and fatigue in patients with AS. METHODS Patients received infliximab 5 mg/kg (n = 188) or placebo (n = 68) at weeks 0, 2, 6, 12, and 18. Hemoglobin, interleukin-6 (IL-6), and C-reactive protein (CRP) levels, fatigue (visual analog scale [VAS]), physical function (Bath Ankylosing Spondylitis Functional Index [BASFI]), and disease activity were evaluated at baseline and week 24. Anemia was defined as a hemoglobin level <12 gm/dl for women and <13 gm/dl for men. RESULTS At baseline, 11 placebo group patients (16.2%) and 37 infliximab group patients (19.7%) had anemia. Of these, more infliximab-treated patients achieved normal hemoglobin levels at week 24 compared with patients receiving placebo (70.3% versus 27.3%; P = 0.0155). Infliximab-treated patients had significant improvements in mean hemoglobin concentration (0.7 gm/dl versus -0.3 gm/dl), BASFI score (-2.1 versus -0.2), and fatigue VAS score (-2.4 versus -0.4) compared with placebo patients (P < 0.001). Multiple regression analyses showed that improvements in hemoglobin level were significantly and independently associated with improvements in physical function and fatigue. Infliximab-treated patients with elevated CRP or IL-6 levels at baseline were more likely than those with low levels to have improvement in hemoglobin levels. CONCLUSIONS Infliximab treatment significantly decreased the proportion of AS patients with anemia and improved hemoglobin levels compared with placebo. Improvement in hemoglobin level was independently associated with improvements in physical function and fatigue.