Yes, research demonstrates a significant effect when using hypnosis for weight loss.
In a 9-week study of two weight management groups (one using hypnosis and one not using hypnosis), the hypnosis group continued to get results in the two-year follow-up, while the non-hypnosis group showed no further results (Journal of Clinical Psychology, 1985).
In a study of 60 women separated into hypnosis versus non-hypnosis groups, the groups using hypnosis lost an average of 17 pounds, while the non-hypnosis group lost an average of only .5 pounds (Journal of Consulting and Clinical Psychology, 1986).
In a meta-analysis, comparing the results of adding hypnosis to weight loss treatment across multiple studies showed that adding hypnosis increased weight loss by an average of 97% during treatment, and even more importantly increased the effectiveness POST TREATMENT by over 146%. This shows that hypnosis works even better over time (Journal of Consulting and Clinical Psychology, 1996).
Here are some of the studies:
Cochrane, Gordon; Friesen, J. (1986). Hypnotherapy in weight loss treatment. Journal of Consulting and Clinical Psychology, 54, 489-492.
Kirsch, Irving (1996). Hypnotic enhancement of cognitive-behavioral weight loss treatments–Another meta-reanalysis. Journal of Consulting and Clinical Psychology, 64 (3), 517-519.
Allison, David B.; Faith, Myles S. Hypnosis as an adjunct to cognitive-behavioral psychotherapy for obesity: A meta-analytic reappraisal. Journal of Consulting and Clinical Psychology. 1996 Jun Vol 64(3) 513-516
Stradling J, Roberts D, Wilson A, Lovelock F. Controlled trial of hypnotherapy for weight loss in patients with obstructive sleep apnoea. International Journal of Obesity Related Metababolic Disorders. 1998 Mar;22(3):278-81.