Abstract Purpose This study aimed to illustrate the possibility of an unfavorable response to treatment with the anabolic agent romosozumab for patients with severe osteoporosis and to discuss explanations for treatment failure. Methods Dual-energy x-ray absorptiometry (DXA) including vertebral fracture assessment (VFA) and X-rays of the thoracolumbar spine was used to assess bone mineral density (BMD) and the presence of vertebral fractures before and after treatment with romosozumab. Results Our patient developed a decrease in the BMD of the hip, two incident new vertebral fractures, and worsening of one prevalent vertebral fracture during 1 year treatment with romosozumab. We have not detected non-adherence, there was no pretreatment with anti-resorptives, and we observed no signs of secondary osteoporosis and/or comorbidities. Conclusion As the number of patients treated with romosozumab is rising, it becomes more likely that more patients will be found with new fractures and/or an unfavorable BMD response. Probably, the unfavorable response is a (bad) chance finding, but we think it is crucial for clinicians and patients to exclude nonadherence, new comorbidities and pretreatment with anti-resorptives as explanation in these patients.