In this study, researchers examined the effect of expressing anger vs. holding it in among 84 chronic low back pain (CLBP) patients. They measured tightness in the lower paraspinal (LP) muscle after harassing the patients and either allowing them to express anger or not. The people who were not allowed to express their anger had the worst muscle tightening and took the longest to recover. The moral of the study—be honest with and express your feelings, or be prepared to carry them around as back pain!
References
Trait Anger Management Style Moderates Effects of Actual ("State") Anger Regulation on Symptom-Specific Reactivity and Recovery Among Chronic Low Back Pain Patients. Psychosom Med 2008, doi:10.1097/PSY.0b013e3181835cb7
Lev, I think that holding in anger effects our whole body, and causes many other illnesses.