All previously reported comparative studies of 511 keV single-photon emission computed tomography (SPECT) and positron emission tomography (PET) have used one fluorine-18-fluorodeoxyglucose (FDG) dose, followed by PET and SPECT on the same day. This approach is inherently biased against the second imaging study. Therefore, we prospectively compared conventional PET and 511 keV SPECT in 23 patients with proven malignancy using separate 370 MBq FDG doses on different days employing an ECAT 951/31R PET scanner and a Trionix XLT-20 for SPECT. Discrepancies were evident in twelve of 23 patients (52%). In eight of these (66%) findings were seen exclusively on PET and represented the only metabolic evidence of disease. Thirty-seven of the 52 lesions (71%) detected at PET were also defined by SPECT, most above 2 cm. In 4 cases of extrahepatic abdominal disease (3 colorectal, 1 melanoma), both PET and SPECT missed small recurrent omental and perivesical lesions; several lesions up to 1.2 cm were also missed by CT and MRI.