OBJECTIVE During a prospective cohort study to determine the effectiveness of and adverse effects associated with transurethral collagen injection for treatment of stress urinary incontinence in women, we observed 3 cases of delayed allergic reaction at the skin test site associated with arthralgia. METHODS A total of 337 women with at least a 1-year history, physical findings and urodynamic abnormalities consistent with stress urinary incontinence, who required pads or protective clothing and who had no or only grade 1 cystocele were enrolled in this prospective cohort study. Adverse effects were documented by a third party at each followup. RESULTS Delayed reaction at the skin test site occurred in 3 patients (0.9%), and was associated with arthralgias in 2. CONCLUSIONS The incidence and systemic nature of this type of reaction suggest that gluteraldehyde cross-linked collagen injection is not as innocuous as previously believed. Patients should be counseled regarding the unknown long-term outcome of this complication. Before treatment clinicians should consider double skin testing.