Positron emission tomography using 18F-fluorodeoxyglucose (FDG-PE) was performed in four patients with non-Hodgkin's lymphoma on the day after initial chemotherapy, in an attempt to predict the effects of chemotherapy earlier than standard methods. Twelve regions displaying intense uptake on baseline FDG-PET were selected, and decreases in the rate for each region were calculated from standardized uptake values on the day following chemotherapy. Seven of the 12 regions demonstrated decrease rates of 60% or more, and two decreased by 100%. This study indicates that FDG-PET on the day after first chemotherapy seems to reflect the effect of chemotherapy on malignant lymphoma.