PET is essential for the post-treatment assessment of lymphomas because a negative PET scan after treatment is required for a complete remission and curative outcome. Functional imaging with 18F-FDG PET enables evaluation of the early metabolic changes rather than the moprphologic changes of the lymphoma occurring later. Among 186 patients diagnosed with Hodgkin's Disease or Non-Hodgkin's Lymphoma who were referred to the PET Center in this institution from April 2002 to June 2010, 35 patients were included in this study. All 14 patients who only had end-of-chemotherapy scans without midcycle scans had no tumor recurrence on subsequent scan/s, with a median follow-up of 17.5 months. Of the remaining 21 patients, 11 patients had positive studies, 9 had negative studies and 1 had an indeterminate study of the midcycle scans. All the patients obtained similar results on the subsequent scans, excluding 5 with no follow-up scans, with median follow-ups of 13 months for the PET-positive patients and 23 months for the PET-negative patients. These results support the strong prognostic value of PET for aggressive lymphomas, whether the imaging is performed at the end of therapy or after only a few cycles of chemotherapy. PET has consistently been shown to identify positive respondents with higher survival probabilities and longer progression-free survival periods from non-responders on subsequent scans.