To assess the testicular Sertoli cell function according to inhibin B levels in ankylosing spondylitis (AS) patients and the possible effect of anti-TNF therapy on this hormone production, 20 consecutive AS patients and 24 healthy controls were evaluated. At study entry, AS patients were not receiving sulfasalazine/methotrexate and never have used biological/cytotoxic agents. They were assessed by serum inhibin B levels, hormone profile, urological examination, testicular ultrasound, seminal parameters, and clinical features. Ten of these patients received anti-TNF treatment and they were reevaluated for Sertoli function and disease parameters at 6 months. Four of them agreed to repeat sperm analysis. At study entry, the median of inhibin B (68 vs. 112.9 pg/mL, p = 0.111), follicle-stimulating hormone levels (3.45 vs. 3.65 IU/L, p = 0.795), and the other hormones was comparable in AS patients and controls (p > 0.05). Sperm analysis was similar in AS patients and controls (p > 0.05) with one AS patient presenting borderline low inhibin B levels. Further analysis at 6 months of the 10 patients referred for anti-TNF therapy, including one with borderline inhibin B, revealed that median inhibin B levels remained stable (116.5 vs. 126.5 pg/mL, p = 0.431) with a significant improvement in C-reactive protein (27.8 vs. 2.27 mg/L, p = 0.039). Sperm motility and concentration were preserved in the four patients who repeated this analysis after TNF blockage. In conclusion, this was the first study to report, using a specific marker, a normal testicular Sertoli cell function in AS patients with mild to moderate disease activity.