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We previously correlated non-Hodgkin's lymphoma (NHL) histology with 18fluoro-2-deoxyglucose–positron emission tomography (FDG–PET) intensity: a standardized uptake value (SUV) >10 predicted aggressive lymphoma with >80% certainty and an SUV >13, with >90% certainty.To evaluate SUV in transformed lymphoma, we identified all FDG–PET scans for NHL at Memorial Sloan-Kettering Cancer 1999–2007 with (i) biopsy-proven transformation, (ii) no therapy 60 days before PET scan and (iii) FDG–PET scans no more than 60 days before or 90 days after transformation.In 5 of 40 patients, the biopsy site was excised before PET; in two, only marrow was biopsied. In the remaining 33 patients, the SUV of the biopsy site ranged from 3 to 38, mean 14, median 12. Eighteen of 33 biopsies (55%) had an SUV >10 and 16 (48%) >13. The highest SUV in a transformed lymphoma PET scan (SUVstudy-max) ranged from 3.2 to 40, mean 15, median 12. Twenty-five of 40 patients (63%) presented with an SUVstudy-max >10 and 20 (50%) >13.Like de novo aggressive lymphomas, the majority of transformations have a high SUVstudy-max for a given pretreatment staging study, although many do not have very high values. Transformation should be suspected in indolent lymphoma with high SUVs on FDG–PET. Biopsies should be directed to the site of greatest FDG avidity.