Epidemiological evidence suggests that diets rich in tomatoes and tomato products may be protective against risk for certain cancers, especially prostate cancer. The carotenoid lycopene has received the most attention as the compound in tomatoes responsible for this noted decrease in cancer risk. In raw, red tomatoes, approximately 95% of the total lycopene is present in the all-trans form. Despite the predominance of dietary all-trans-lycopene, cis-isomers account for 58-73% of total lycopene in human serum, and a surprisingly high 79-88% in benign or malignant prostate tissue. This observation is related to isomerization in vivo, as demonstrated by conventional and tracer...