A high performance positron emission tomograph, HEADTOME III, with 480 bismuth germanate oxide detectors (13.4 X 25.0 X 40.0 mm), arranged in three rings of 750 mm diameter, with independent shadow masks and septa, is described. Image plane resolution at the center of the field of view (FOV) is 7.9 and 6.5 mm full width at half maximum (FWHM) in low resolution (LR) and high resolution (HR) mode, respectively. Axial resolution is 13.1, 11.3, and 9.1 mm FWHM at the center of FOV for direct planes of low quantitation (LQ), cross planes of LQ, and high quantitation (HQ) mode, respectively. Sensitivities evaluated by true events for a 68Ga 20 cm diameter cyclindrical pool were 34.5 (56.7), 16.4 (27.5), 19.4 and 9.5 kcps (muCi/ml)-1 with the direct (cross) planes of LQLR, direct (cross) planes of LQHR, and direct planes of HQLR and HQHR, respectively. The fraction of scattered coincidence in a cold spot phantom after software correction is 2.5% in LRLQ mode. Count rate linearity after software correction is within 1% up to 50 X 10(3) true events per second per plane with LRLQ mode.