The viability and biodistribution of 68Ga-mercaptopyridine-N-oxide (MPO)-labelled autologous platelets was studied in 10 patients. The average platelet labelling yield was 36 ± 12% and injected activity was 2.0 ± 0.9 mCi 68Ga. The % activity in platelets per ml whole blood was 64 ± 20% at 15 min-1.0 h postinjection and 76 ± 14% at 2–4 h. The average recovery of platelets (% injected platelets circulating in peripheral blood) was 31 ± 21% at 15 min-1 h and 39 ± 20% at 2–4 h. The positron emission tomographic (PET) images showed high circulating vascular background. Two patients had technically inadequate scans, and six were false negative due to high blood background. One patient with a massive pulmonary embolus occurring 24 h prior to scanning had marked uptake of 68Ga platelets in a large clot in the superior branch of the right main pulmonary artery. A second patient, with 68Ga platelets circulating during angioplasty of a left posterior tibial artery stenosis, had intense uptake in the lesion shown on the PET scan obtained 4 h following the procedure. These results indicate good viability of 68Ga-MPO-labelled autologous human platelets, but poor visualization of clots by PET imaging, due to the high blood background at early times.