We developed positron emission tomography (PET)/computed tomography (CT) viewing software (PETviewer) that can display co-registered PET and CT images obtained by PET/CT and stored on picture archiving and communication systems (PACS). PETviewer has tools for presetting windows for CT display; control bars for PET window level; zoom, pan, and pseudo-color functions; and allows the user to draw a rectangular region of interest (ROI) for standardized uptake value (SUV) measurement. SUV was calculated using PET DICOM header information and the pixel intensity in PETviewer. Reconstructed datasets of PET/CT and maximum intensity projection (MIP) of the PET images were transferred and archived in PACS. Phantom experiments were performed to evaluate the validity of image fusion. PET/CT images were displayed on an independent window in PACS. Transaxial PET images were reformatted as sagittal and coronal PET images, which were displayed with the corresponding CT and PET/CT fusion images with adjustable color and transparency. Transaxial, sagittal, and coronal PET images corresponding to the location of the cursor were shown using cine display of MIP images. All images were displayed in PETviewer within 20 s on a personal computer for PACS, which was equipped with a P4, 1.3-GHz CPU, and 512 Mb of RAM. We could measure maximum and mean SUV in a ROI using PETviewer. Transaxial fused images of patients and phantoms showed excellent registration and fusion of PET and CT images in the X and Y directions. PETviewer provided very useful clinical tools for assessing PET/CT images on PACS and should assist in maximizing the benefits derived from PET/CT imaging.