Characterisation of the physical performance of the new integrated PET/CT system Discovery ST (GE Medical Systems) has been performed following the NEMA NU 2-1994 (N-94) and the NEMA NU 2-2001 (N-01) standards in both 2D and 3D acquisition configuration. The Discovery ST combines a four or eight multi-slice helical CT scanner with a PET tomograph which consists of 10,080 BGO crystals arranged in 24 rings. The crystal dimensions are 6.36.330mm3 and they are organised in blocks of 66 crystals, coupled to a single photomultiplier tube with four anodes. The 24 rings of the PET system allow 47 images to be obtained, spaced by 3.27mm, and covering an axial field of view of 157mm. The low- and high-energy thresholds are set to 375 and 650keV, respectively. The coincidence time window is set to 11.7ns. Using the NEMA N-94 standard, the main results were: (1) the average (radial and tangential) transverse spatial resolution (FWHM) at 1, 10 and 20cm off axis was 6.28mm, 7.09mm and 7.45mm in 2D, and 6.68mm, 7.72mm and 8.13mm in 3D; (2) the sensitivity for true events was 8,567cps/kBq/cc in 2D and 36,649 cps/kBq/cc in 3D; (3) the scatter fraction was 15% in 2D and 30% in 3D; (4) the peak true events rate, the true events rate at 50% of the system dead-time and the true events rate when equal to the random events rate were 750kcps at 189.81kBq/cc, 744kcps at 186.48kBq/cc and 686kcps at 150.59kBq/cc, respectively, in 2D, and 922kcps at 44.03kBq/cc, 834kcps at 53.28kBq/cc and 921kcps at 44.03kBq/cc in 3D; (5) the noise equivalent count (NEC) peak rate was 270kcps at 34.38kBq/cc in 3D, with random coincidences estimated by delayed events. Using the NEMA N-01 standards the main results were: (1) the average transverse and axial spatial resolution (FWHM) at 1cm and 10cm off axis was 6.28 (4.56)mm and 6.88 (6.11)mm in 2D, and 6.29 (5.68)mm and 6.82 (6.05)mm in 3D; (2) the average sensitivity for the two radial positions (r=0cm and r=10cm) was 1.93cps/kBq in 2D and 9.12cps/kBq in 3D; (3) the scatter fraction was 19% in 2D and 45% in 3D; (4) the NEC peak rate was 54kcps at 46.99kBq/cc in 2D and 45.5kcps at 10.84kBq/cc in 3D, when random coincidences were estimated by using k=2 in the NEC formula, while the NEC peak rate was 81kcps at 64.43kBq/cc and 66kcps at 14.86kBq/cc in 2D and 3D, respectively, when random coincidences were estimated by using k=1 in the NEC formula. The new integrated PET-CT system Discovery ST has good overall performances in both 2D and 3D, with in particular a high sensitivity and a very good 3D NEC response.