Abstract Avalanche photodiodes (APD) offer advantages in terms of weak scintillation detection, fast time response, and magnetic field insensitivity. We have developed new types of large-area, reverse-type APD-arrays specifically designed for high-resolution positron emission tomography (PET). Each device has a monolithic 16 × 16 (or 8 × 8 ) pixel structure with an active area of 1.0 (or 4.0, 0.25) mm 2 for each pixel. An excellent gain uniformity ( ⩽ 10 % ) and low dark noise ( ⩽ 0.3 nA ) have been achieved, measured at room temperature. Energy resolution of 7.2 % (FWHM) was obtained for the direct detection of 5.9 keV X-rays, while 10.2 % (FWHM) was obtained for 662 keV gamma-rays when coupled with a LYSO scintillator matrix. An excellent time resolution of 102 ps (FWHM) was obtained for a monolithic, 3 mm φ APD pixel. These results suggest that APD-arrays could be a promising device for future applications in nuclear medicine.