A prospective study was performed, comparing gallium scintigraphy (67Ga) and positron emission tomography (PET) using fluorine-18 fluorodeoxyglucose (18FDG), to monitor the response of aggressive non-Hodgkin's lymphoma during treatment. 67Ga and 18FDG scans were performed in 26 patients after two cycles of CHOP (cyclophosphamide, doxorubicin, vincristine, prednisone) therapy. The scans were reviewed independently by four experienced nuclear physicians, who were blinded for the alternative scan technique and follow-up. Eleven out of 26 patients remained free from progression with a mean follow up of 25 +/- 5 months, whereas 14 patients relapsed, and one died of lung cancer. Interobserver variation was significantly greater for 67Ga than for 18FDG PET. Some 64% of patients who had a negative early restaging 18FDG PET remained free from progression versus 50% of patients with negative 67Ga scans. Only 25% of patients with a positive PET remained disease free versus 42% of 67Ga-positive patients. Time to progression was associated with 18FDG PET results, but not with those by 67Ga. 18FDG PET had better test characteristics than 67Ga for the evaluation of early response in aggressive non-Hodgkin's lymphoma patients.