Objective: To determine the immediate effects on both pain and active range of motion (ROM) of the unilateral posteroanterior (PA) mobilization technique on the painful side in mechanical neck pain patients presenting with unilateral symptoms.Design: Triple-blind, randomized controlled trial.Setting: Outpatient physical therapy, institutional clinic.Participants: Patients (N=60), 2 physical therapists, and 1 assessor involved in this study.Interventions: The patients were randomly allocated into either preferred or random mobilization group by using an opaque concealed envelope. The first therapist performed the screening, assessing, prescribing the spinal level(s), and the grade of mobilization. The second therapist performed the mobilization treatment according to their allocated group stated in an envelope. The assessor who was blind to the group allocation conducted the measurements of pain and active cervical ROM.Main Outcome Measures: Pain intensity, active cervical ROM, and global perceived effect were measured at baseline and 5 minutes posttreatment.Results: After mobilization, there were no apparent differences in pain and active cervical ROM between groups. However, within-group changes showed significant decreases in neck pain at rest and pain on most painful movement (P=0.001) with a significant increase in active cervical ROM after mobilization on most painful movement (P=0.002). Conclusions: The results of this study did not provide support for the preference of the unilateral PA mobilization on the painful side to the random mobilization.