Objectives. To define what expert clinicians consider a dramatic response in ankylosing spondylitis (AS) patients treated with biologic therapies based on patient and physician assessments of global disease activity. To compare this expert clinician-derived criteria to the Ankylosing Spondylitis Assessment Study (ASAS) Group criteria for improvement. Methods. Forty consecutive AS patients were treated in a 1-year open-label protocol with infliximab. Response to treatment at week 52 was defined using ASAS response criteria. For the purpose of this exercise, improvement using ASAS criteria was defined by consensus among experts as good with 50% improvement from baseline (ASAS(50)) and dramatic with 70% improvement from baseline (ASAS(70)). Experts established separate criteria for improvement in disease activity as good or dramatic based on patient and physician global assessment of disease activity. Results. Twelve of 40 patients met the ASAS(70) criteria, however, only, 8 met the expert definition of a dramatic improvement based on physician global scores and 5 met the expert definition of a dramatic improvement based on patient global assessment of disease activity. Agreement was poor between ASAS(50) or ASAS(70) and expert definition of improvement based on physician global scores (kappa < 0.3), but agreement was moderate to good between ASAS(50) or ASAS(70) and expert definition of improvement based on patient global scores, (kappa = 0.6-0.7). Conclusions. Differential response experienced by AS patients treated with infliximab was adequately captured by the ASAS composite improvement criteria. Overall, this study demonstrates the validity of the ASAS criteria for the detection of improvement in AS patients treated with biologics. However, the patient global assessment of disease activity may be sufficient to monitor changes in disease activity in these patients.