Objective. To evaluate the effect of low body mass index (BMI) on blood loss during primary total hip arthroplasty (THA) in ankylosing spondylitis (AS) patients. Methods. Two hundred seventy-seven consecutive AS patients who underwent primary THA were retrospectively studied. The patients were divided by BMI into an underweight group (BMI < 18.5 kg/m2) and a normal weight group (18.5 kg/m2 < BMI < 25 kg/m2). Demographics, perioperative laboratory values, intraoperative data, blood loss, transfusion rate, transfusion reactions, surgical complications, hospitalization cost, and length of stay (LOS) were collected and analyzed. Results. Of 277 AS patients, 236 were eligible for inclusion in the study. A total of 91 (39%) patients were underweight. The hidden blood loss, transfusion rate, transfusion reactions, and hospitalization cost in the underweight group were significantly higher than those in the normal weight group. Conclusions. For AS patients, BMI appears to be correlated with blood loss during primary THA. Compared with patients of normal weight, low BMI patients have the potential to suffer more postoperative hidden blood loss and to require a higher transfusion rate.