The present invention relates to methods of passive infrared imaging and dynamic infrared imaging of a tumor or lesion (“DIR”) where multiple images are taken before, during and after heating or cooling the tumor or lesion. Patterns of intensity of infrared emission in the infrared image(s) are related to the level of vascularity or metabolic activity of the tumor or lesion, and are used to identify size and shape of the tumor or lesion. The patterns can also be used to determine whether Kaposi's sarcoma lesions, skin cancer lesions and melanomas are active or inactive. Changes in the patterns of infrared emission determined by comparing images taken before, during and after the onset of drug or radiation therapy, are used to assess efficacy of the therapy. The methods can also be used to study tissue affected by angiogenic diseases.