The purpose of the present study was to assess if small animal PET is useful for serially monitoring the development of a human anaplastic large cell lymphoma (ALCL) murine xenograft and for the early selection of tumour bearing animals. The human ALCL Karpas 299 cell line was subcutaneously injected in 6-week-old NOD/SCID (non-obese diabetic/NCrCrl- Prkdc) mice (10(7) cells/mouce in 150 pil FBS) at the right flank level. Small animal 18F-fluorodeoxyglucose positron emission tomography (18F-FDG PET) was serially performed (intravenous injected dose: 20 MBq in < 0.15 ml, uptake time: 60 min, image acquisition: 1 bed position of 15 min): early PET at 2 days after cell inoculation in 4/8 mice and at 4 days in the remainig 4/8, later PET scans were performed in all the animals at 7, 14, 21 and 28 days after inoculation. The images were evaluated visually and the tumour to background ratio (TBR) was used for semiquantitative analysis. Pathology sections were obtained in all cases. PET detected the presence of the tumour as early as seven days after inoculation in 4/8 mice and at 14 days in 2/8. Of the two remaining mice, one died after the first PET scan (thus preventing any evaluation of detection time) while the other showed a microscopic neoplastic infiltration at tracheal level at autopsy. Mean TBR progressively increased in all positive cases, particularly in the first 3 weeks, reaching a plateau afterwards. PET was positive in 6/8 (75%) animals, detecting the presence of viable tumour cells earlier than macroscopic evaluation, thus may be used for the early identification of tumour bearing animals.