One early effect of the treatment of tumours by the new modality photodynamic therapy (PDT) is a reduction in tumour glucose levels. We have employed the widely used positron-emitting glucose analogue flurorine-18 fluoro-2-deoxy-D-glucose ([18F]-FDG), to determine whether, in principle, PDT-induced injury might be delineated non-invasively and quantitatively by positron emission tomography (PET). The scanner was of the high-density avalanche-chamber (HIDAC) type with a resolution of 2.6 mm. Subcutaneous T50/80 mouse mammary tumours, sensitised by haematoporphyrin ester, were illuminated by graded doses of interstitial 630 nm light. Thirty hours later, any remaining viable tumour was detected (a) by region-of-interest analysis of the PET images and (b) by gamma counting the excised tumour. PET measurements of % uptake of [18F]-FDG into tumour correlated closely with ex vivo gamma counting (slope=0.976, r2=0. 995), validating the in situ method. Uptake into untreated, control tumours was 3.8%+/-1.1% of the injected activity. Uptake of [18F]-FDG into treated tumours decreased by 0.7% for every 100 mm3 reduction in remaining viable histological volume. Outcome was further compared with that measured by (a) T2-weighted proton imaging on a 4.7-T magnetic resonance imaging (MRI) system and (b) histological analysis of subsequently sectioned tumours. PET using [18F]-FDG described the absolute volume of surviving tumour histological mass to the same degree as high-resolution MRI. The conclusion of these initial studies is that PET with [18F]-FDG, although non-specific, quantitatively described at early times the extent of tumour destruction by PDT.