Post-mastectomy chronic lymphedema as a complication of breast cancer treatment is primarily managed with Complete Decongestive Therapy (CDT). We examined various factors for correlating with results of Phase I CDT treatment in controlling the upper extremity lymphedema. Study population consisted of patients with lymphedema referred to the Lymphedema Clinic of the Iranian Breast Cancer Research Center for control of arm edema. After obtaining the demographic and clinical data, patients were treated with CDT for 2 - 3 weeks. One hundred and thirty seven patients (mean age +/- SD; 53.5 +/- 10 years) were studied. In 48.7% of patients, the affected arm was the dominant limb. Fifty percent of patients experienced lymphedema during the first year after surgery, and mean duration of lymphedema was 35 +/- 43 months. Mean volume reduction was 43% +/- 14.87% (p = 0.03). There was a significant relationship between the percent of volume reduction and initial lymphedema volume (p=0.003) as well as duration of lymphedema (p=0.002). Our results demonstrate that Phase I CDT treatment is very effective for post mastectomy lymphedema, and particularly if it is provided in earlier stages of disease. In addition, CDT also has an important role in reducing clinical symptoms and improving limb function. In the appropriate setting, Phase I CDT has been an effective method of controlling post mastectomy lymphedema in this Iranian population.