18F-Fluorodeoxyglucose-positron emission tomography (FDG-PET) plays an increasing role in the management of patients with lymphoma, for which it is successfully used for staging and treatment monitoring. We report seven patients with a history of lymphoma who presented a positive FDG-PET suggestive of lymphoma relapse and for which FDG-PET oriented biopsies revealed alternative diagnoses. Early in lymphoma follow-up, persistence of focal increased FDG activity corresponded to inflammatory or infectious lesions in two patients: one aspergillosis and one sarcoidosis. Later in the follow-up, five cases of secondary malignancies were identified (three lung cancers, one epidermoid carcinoma, and one villous tumor) in this particularly exposed population. The routine use of FDG PET to evaluate lymphoma significantly increases the probability of detecting unexpected diseases. These cases illustrate the potential pitfalls in PET follow-up. Because FDG is not lymphoma-specific, a relapse suspected only on FDG-PET imaging requires biopsy, as alternative diagnoses--infectious or malignant--are possible. Our data draws clinician's attention to potential false-positive FDG-PET findings, which may lead to therapeutic mistakes. Our data also suggests that FDG-PET might be a new imaging modality for long-term monitoring of late effects, especially second cancer occurrence.