Abstract Background: Malignant lymphoma is the most common hematological malignancy accounts for approximately 8% of all adult malignancies. Lymphomas are broadly divided into Hodgkin lymphoma and non-Hodgkin's lymphoma. Non-Hodgkin lymphoma accounts for about 5% of all cases of cancer. Non-Hodgkin lymphomas have the vast majority of cases and have a greater predilection to disseminate to extra-nodal sites. Aim of Study: The aim of this work is to study the role of PET/CT in the diagnosis and follow-up of Non-hodgkin lymphoma and assess its extranodal extention. Patients and Methods: The study was conducted on thirty patients where the diagnosis of non-hodgkin lymphoma have been pathologically confirmed, 18 males (60%) and 12 females (40%), their ages are ranged between 19 to 73 years old. All patients were subjected to full history taking, laboratory testing, biopsy and histopathology, the studied cases according to pathalogical type of non-Hodgkin lymphoma mostly were of B-cell type (80%) and (20%) T-cell type, CT scan and PET/CT examination. All patients were examined using Siemens Bio-graph true point PET/CT scanner. Results: Extranodal involvement was detected by PET/CT in 16 patients (53.3%). Their distribution was as follows; head and neck: 4 patients (13.3%), lung: 3 patients (10%), abdomen: 12 patients (40%), MSK: 10 patients (33.3%), while CECT detected 8 patients had extra nodal involvement their distribution was as follow; head and neck: 2 patients (6.6%), lung: 1 patient (3.3%), abdomen:7 patients (43.7%), MSK: 6 patients (16.6%). Thirty patients referred for initial assessment SUV max among the 30 patients referred for initial assessment ranged from 3.7-34.0 with mean value of 16.91±8.07 SD. In aggressive type of NHL SUVmax ranged from 10.5- 34.0 with mean value of 19.9±6.65 SD, the aggressive type of NHL is significantly higher (p£0.01) than in indolent type. The cases were staged according to the size in CECT and FDG activity in PET/CT; CECT detected 6 patients (20%) in stage I, 9 patients (29.3%) in stage II, 7 patients (23.3%) in stage III and 8 patients (26.7%) in stage IV. The PET/CT detected 4 patients (13.3%) in stage I, 8 patients (26.7%) in stage II, 5 patients (16.7%) in stage III and 13 (43.3%) patients in stage IV, therefore PET/CT was statistically significant at p£0.0004 in the initial staging. In follow-up assessment among the 20 patients referred after treatment; SUVmax among the 20 patients referred for follow-up assessment, their SUVmax of the initial PET/CT examination ranged from 3.7-34.0 with a mean value of 17.57±  8.54 SD, with a significant change (p£0.046) in their follow-up examination with SUVmax ranged from 0.0-28.0 with a mean value of 11.73±9.25 SD. CECT showed 4 patients (20%) in complete regression, 13 patients (65%) in stable disease and 1 patient (5%) in progressive disease while PET/CT showed 7 patients (35%) in complete remission, 4 patients (20%) in partial remission, 0 (0.0%) patients in stationary disease, 9 (45%) patients in progressive disease. Thus, PET/CT was concordant with CECT in 6 (30%) cases and disagreed with CECT in 14 (70%) cases with significance (p£0.015). Conclusion: PET/CT agreed with CECT in the initial staging of 13 (43.3%) cases while disagreed with CECT in initial staging of 17 (56.7%) cases, where PET/CT upstaged 13 (43.3%) cases and down staged 4 (13.3%) cases. In follow-up PET/CT was concordant with CECT in 9 (45%) cases and disagreed with CECT in 14 (55%) cases with significance (p£0.034).