PURPOSE: This study was conducted to investigate the effect of the sling exercise wearing a neck orthosis on the craniovertebral angle, muscle tension, and headaches in adults with a forward head posture and tension headache. METHODS: In this single-blinded, randomized, controlled, comparative study, a total of 22 adults with forward head postures and tension headaches were randomly assigned the experimental group (sling exercise wearing a neck orthosis, n = 11) or the control group (sling exercise without a neck orthosis, n = 11). All participants undertook the sling exercise program (3×/week for 4 weeks). The craniovertebral angle, muscle tension, and headache were measured before and after the 4-week training. RESULTS: Significant improvements were observed in the craniovertebral angle, muscle tension, and headache in the experimental group (p < .05). This group also showed a larger decrease in the muscle tension and headache (upper trapezius, -4.97 Hz vs -1.70 Hz, p < .05; splenius capitis, -5.44 Hz vs -2.54 Hz, p < .05; headache, -19.73 score vs -14.64 score, p < .05, respectively). CONCLUSION: The sling exercise wearing a neck orthosis could be an effective way to relieve the symptoms caused by a forward head posture. It could also be a more effective way of decreasing muscle tension and headaches than the sling exercise without wearing a neck orthosis.