We assessed the quality of current evidence on fluorine-18 fluorodeoxy glucose positron emission tomography (FDG-PET) performed after a few cycles of chemotherapy for patients with advanced-stage Hodgkin lymphoma (HL) or diffuse large B-cell lymphoma (DLBCL) based on a recently published systematic review of the literature. There is a moderate level of evidence suggesting that interim PET has an excellent prognostic ability to predict treatment failures in low- to intermediate-risk advanced-stage HL patients. Evidence is insufficient for DLBCL due to the clinical heterogeneity of the primary studies. Interim PET should at present not be regarded as an established procedure and it still remains as an unproven test for routine clinical practice. Its use should currently be reserved for research studies where treatment strategies and imaging protocols are standardized.