Purpose: To pilot and determine the feasibility of estimating adverse events in patients with neck pain treated with cervical manipulation/mobilization by Canadian orthopaedic manual physiotherapists (OMPTs) using an online data-collection system to provide estimates for a future larger multi-centre international study. Methods: In a prospective multi-site two-group cohort study, participants received usual care and either (a) combined manipulation and mobilization or (b) mobilization only. Study feasibility objectives and criteria for success were set a priori. Data were analyzed using descriptive statistics. Results: A total of 20 patients were recruited from 6 of 16 participating centres, 17 to the mobilization group and 3 to the combined group. Barriers to data collection included low recruitment, difficulties in using the online data-collection system, and clinicians' and patients' being too busy to participate. Missing data for the primary outcome averaged 28.5%. A total of 69 symptom occurrences were reported during the treatment phase, all benign and transient. Most began within 0–12 hours after treatment (66/69, 95.6%) and lasted for 0–2 days (56/69, 81.2%). Kappa estimates of agreement between therapists and patients on reporting of adverse symptoms across visits 1–5 was substantial (κ=0.68; 95% CI, 0.52–0.84; p<0.01). Conclusions: This pilot study demonstrates substantial challenges in conducting a large multi-centre trial. Brief, benign, and transient adverse events were common; no substantial adverse events were observed.