F-18 deoxyglucose positron emission tomography (FDG PET) is a powerful metabolic imaging tool in the diagnosis of malignancy, staging of disease, detection of residual or recurrences, measurement of the response to therapy, and identification of the site of disease. Despite of the fact that maximal standardized uptake value (SUV) has been applied to measure tumor’s uptake on FDG PET, characterization of a lesion or areas of normal mediastinal and/or liver activity has been used for a comparison with the FDG uptake of the focal lesion(s). Thus, a FDG PET study, for example, is searching or isolating an abnormal “lesion” from a normal background: a “hot” area of abnormal lesion against the relatively “cold” background or a “cold” lesion against the relatively “hot” background. The concept of the principal fits the theory of opposites, which later became “Yin” and “Yang” in the Tai-Chi symbol of I Ching. We reviewed 64 patients’ FDG PET who had been referred for pulmonary mass(es) or nodule(s) on chest radiography and/or CT. Taking the Yin-Yang concept, the Tai-Chi symbol can be applied in FDG PET diagnostic interpretation searching for abnormal lesion(s) as either a cold or hot area in normal or homogenous opposite background. Most malignant lesions are a “hot or Yang” area in the light background–a lesion or lesions against white black background and, a “cold or Yin” lesion in the relatively black background is mostly nonmalignant. In another word, nuclear medicine interpretation is basically searching a Yin (cold) lesion in dark background or a Yang (hot) lesion against bright background as our FDG-PET illustrated.