OBJECTIVES:  Serum CTX-1 (marker of osteoclastic activity) levels have demonstrated to be inversely related to drug efficacy in the suppression of bone resorption. A reference threshold of optimal suppression is yet to be determined. Our goal was to assess CTX-1 level fluctuations in time in a cohort of patients under antiresorptive therapy after a fragility fracture. CTX-1 levels were also compared in patients reporting to be persistent or not and we described suppression profiles according to two CTX-1 thresholds. Our second objective was to assess CTX-1 level variations in time according to fracture sites. METHODS:  A prospective systematic follow-up of patients with fragility fractures through a Fracture Liaison Service (FLS) includes 543 patients. We analyzed data of CTX-1 measured at baseline, 6, 12 and 18 months of follow-up in the FLS. Reported-persistence to treatment was correlated with levels of CTX-1, and proportions of patients suppressed with CTX-1 ≤ 0.3 and of ≤ 0.2ng/mL thresholds were obtained at 6, 12 and 18 months. Mean CTX-1 values were measured at baseline, 6, 12 and 18 months according to three categories of fracture; hip, vertebral and non-hip/non-vertebral (NHNV). RESULTS:  CTX-1 levels were significantly lower at 6, 12 and 18 months compared to baseline (p<0.001). Persistent patients had lower CTX-1 levels than non-persistent patients at 6, 12 and 18 months (p<0.05). Between 80-100% of persistent patients had a CTX-1 level under 0.3ng/ml, 65-75% being under 0.2ng/ml. Mean CTX-1 levels were significantly lower during follow-ups compared to baseline only in NHNV fractures (p<0.001), the number of patients in hip and vertebral fractures groups not elevated enough to obtain powered results. CONCLUSIONS:  CTX-1 levels in fragility fracture patients were consistent with treatment initiation, even more in persistent patients. These results support the use of CTX-1 for bone resorption suppression monitoring in patients treated for a fragility fracture.